2seventy bio (TSVT) 10-K7 Mar 242023 FY Annual reportFinancial data

Company Profile

Cover

Cover - USD ($)	12 Months Ended
Cover - USD ($)	Dec. 31, 2023	Feb. 29, 2024	Jun. 30, 2023
Cover [Abstract]
Document Type	10-K
Document Annual Report	true
Document Period End Date	Dec. 31, 2023
Current Fiscal Year End Date	--12-31
Document Transition Report	false
Entity File Number	001-40791
Entity Registrant Name	2seventy bio, Inc.
Entity Incorporation, State or Country Code	DE
Entity Tax Identification Number	86-3658454
Entity Address, Address Line One	60 Binney Street
Entity Address, City or Town	Cambridge
Entity Address, State or Province	MA
Entity Address, Postal Zip Code	02142
City Area Code	617
Local Phone Number	675-7270
Title of 12(b) Security	Common Stock, par value $0.0001 per share
Trading Symbol	TSVT
Security Exchange Name	NASDAQ
Entity Well-known Seasoned Issuer	No
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Filer Category	Non-accelerated Filer
Entity Small Business	true
Entity Emerging Growth Company	true
Entity Ex Transition Period	false
ICFR Auditor Attestation Flag	false
Document Financial Statement Error Correction [Flag]	true
Document Financial Statement Restatement Recovery Analysis [Flag]	false
Entity Shell Company	false
Entity Public Float			$ 508,404,846
Entity Common Stock, Shares Outstanding		51,311,132
Documents Incorporated by Reference	The registrant intends to file a definitive proxy statement pursuant to Regulation 14A relating to the 2024 Annual Meeting of Stockholders within 120 days of the end of the registrant’s fiscal year ended December 31, 2023. Portions of such definitive proxy statement are incorporated by reference into Part III of this Annual Report on Form 10-K to the extent stated herein.
Amendment Flag	false
Entity Central Index Key	0001860782
Document Fiscal Year Focus	2023
Document Fiscal Period Focus	FY

Audit Information

Audit Information	12 Months Ended
Audit Information	Dec. 31, 2023
Audit Information [Abstract]
Auditor Firm ID	42
Auditor Name	Ernst & Young LLP
Auditor Location	Boston, Massachusetts

Consolidated Balance Sheets

Consolidated Balance Sheets - USD ($) $ in Thousands	Dec. 31, 2023	Dec. 31, 2022
Current assets:
Cash and cash equivalents	$ 74,958	$ 71,032
Marketable securities	142,031	195,238
Prepaid expenses	7,365	13,652
Receivables and other current assets	13,411	20,960
Total current assets	237,765	300,882
Property, plant and equipment, net	58,150	55,735
Marketable securities	4,816	1,414
Intangible assets, net	6,594	7,302
Goodwill	0	12,056
Operating lease right-of-use assets	219,958	240,885
Restricted investments and other non-current assets	38,143	38,391
Total assets	565,426	656,665
Current liabilities:
Accounts payable	6,028	7,208
Accrued expenses and other current liabilities	25,688	54,678
Operating lease liability, current portion	12,660	11,164
Deferred revenue, current portion	15,403	3,000
Collaboration research advancement, current portion	0	3,744
Total current liabilities	59,779	79,794
Deferred revenue, net of current portion	3,918	5,000
Operating lease liability, net of current portion	244,013	259,008
Other non-current liabilities	2,416	2,397
Total liabilities	310,126	346,199
Commitments and contingencies (Note 8)
Stockholders’ equity:
Preferred stock, $0.0001 par value; 10,000 shares authorized, 0 shares issued and outstanding at December 31, 2023 and December 31, 2022	0	0
Common stock, $0.0001 par value; 200,000 shares authorized, 50,632 and 37,928 shares issued and outstanding at December 31, 2023 and December 31, 2022 respectively	5	4
Additional paid-in capital	766,716	606,986
Accumulated other comprehensive loss	(204)	(2,877)
Accumulated deficit	(511,217)	(293,647)
Total stockholders’ equity	255,300	310,466
Total liabilities and stockholders’ equity	$ 565,426	$ 656,665

Consolidated Balance Sheets (Pa

Consolidated Balance Sheets (Parenthetical) - $ / shares	Dec. 31, 2023	Dec. 31, 2022
Statement of Financial Position [Abstract]
Preferred stock, par value per share (in dollars per share)	$ 0.1000	$ 0.1000
Preferred stock, shares authorized (in shares)	10,000,000	10,000,000
Preferred stock, shares issued (in shares)	0	0
Preferred stock, shares outstanding (in shares)	0	0
Common stock, par value (in dollars per share)	$ 0.0001	$ 0.0001
Common stock, shares authorized (in shares)	200,000,000	200,000,000
Common stock, shares issued (in shares)	50,632,000	37,928,000
Common stock, shares outstanding (in shares)	50,632,000	37,928,000

Consolidated and Combined State

Consolidated and Combined Statements of Operations and Comprehensive Loss - USD ($) shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Revenue:
Collaborative arrangement revenue	$ 71,601	$ 32,358	$ 26,921
Total revenues	100,387	91,496	54,522
Operating expenses:
Research and development	230,758	248,735	252,617
Cost of manufacturing for commercial collaboration	14,819	14,851	9,320
Selling, general and administrative	69,414	79,450	93,506
Share of collaboration loss	0	9,642	10,071
Restructuring expenses	8,614	0	0
Cost of royalty and other revenue	2,099	1,726	2,517
Change in fair value of contingent consideration	235	232	439
Goodwill impairment charge	12,056	0	0
Total operating expenses	337,995	354,636	368,470
Loss from operations	(237,608)	(263,140)	(313,948)
Interest income, net	12,413	2,932	88
Other income, net	7,625	6,055	21,647
Loss before income taxes	(217,570)	(254,153)	(292,213)
Income tax expense	0	0	0
Net loss	$ (217,570)	$ (254,153)	$ (292,213)
Net loss per share - basic (in dollars per share)	$ (4.42)	$ (7.13)	$ (12.44)
Net loss per share - diluted (in dollars per share)	$ (4.42)	$ (7.13)	$ (12.44)
Weighted-average number of common shares used in computing net loss per share - basic	49,276	35,637	23,499
Weighted-average number of common shares used in computing net loss per share - diluted	49,276	35,637	23,499
Other comprehensive loss:
Other comprehensive income (loss), net of tax benefit (expense) of $0.0 million, $0.0 million, and $0.0 million for the years ended December 31, 2023, 2022 and 2021, respectively.	$ 2,673	$ (2,165)	$ (712)
Comprehensive loss	(214,897)	(256,318)	(292,925)
Service revenue
Revenue:
Revenue	24,144	55,489	21,381
Royalty and other revenue
Revenue:
Revenue	$ 4,642	$ 3,649	$ 6,220

Consolidated and Combined Sta_2

Consolidated and Combined Statements of Operations and Comprehensive Loss (Parenthetical) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Income Statement [Abstract]
Other Comprehensive Income (Loss), Tax	$ 0	$ 0	$ 0

Consolidated and Combined Sta_3

Consolidated and Combined Statements of Stockholders' Equity - USD ($) $ in Thousands	Total	Common stock	Net parent investment	Additional paid-in capital	Accumulated other comprehensive loss	Accumulated deficit
Beginning balance (in shares) at Dec. 31, 2020		0
Beginning balance at Dec. 31, 2020	$ 74,629	$ 0	$ 74,629	$ 0	$ 0	$ 0
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Stock-based compensation - bluebird allocation	44,626		44,626
Transfers from bluebird bio	517,706		517,936		(230)
Consummation of spin-off transaction (in shares)		23,369,000
Consumption of spin-off transaction	0	$ 2	(384,472)	384,470
Vesting of restricted stock units (in shares)		7,000
Stock-based compensation	9,012			9,012
Issuance of unrestricted stock awards to settle accrued employee compensation (in shares)		209,000
Issuance of unrestricted stock awards to settle accrued employee compensation	6,544			6,544
Other comprehensive income (loss)	(482)				(482)
Net loss	(292,213)		(252,719)			(39,494)
Ending balance (in shares) at Dec. 31, 2021		23,585,000
Ending balance at Dec. 31, 2021	359,822	$ 2	0	400,026	(712)	(39,494)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Vesting of restricted stock units (in shares)		374,000
Stock-based compensation	41,040			41,040
Exercise of stock options (in shares)		2,000
Exercise of stock options	17			17
Issuance of common stock in private placement or public offering, net of issuance costs (in shares)		13,934,000
Issuance of common stock in private placement or public offering, net of issuance costs	165,533	$ 2		165,531
Purchase of common stock under ESPP (in shares)		33,000
Purchase of common stock under ESPP	372			372
Other comprehensive income (loss)	(2,165)				(2,165)
Net loss	$ (254,153)					(254,153)
Ending balance (in shares) at Dec. 31, 2022	37,928,000	37,928,000
Ending balance at Dec. 31, 2022	$ 310,466	$ 4	0	606,986	(2,877)	(293,647)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Vesting of restricted stock units (in shares)		604,000
Stock-based compensation	32,159			32,159
Exercise of stock options (in shares)		1,000
Exercise of stock options	7			7
Issuance of common stock in private placement or public offering, net of issuance costs (in shares)		10,870,000
Issuance of common stock in private placement or public offering, net of issuance costs	117,004	$ 1		117,003
Issuance of common stock to Regeneron (in shares)		1,115,000
Issuance of common stock to Regeneron	9,859			9,859
Purchase of common stock under ESPP (in shares)		114,000
Purchase of common stock under ESPP	702			702
Other comprehensive income (loss)	2,673				2,673
Net loss	$ (217,570)					(217,570)
Ending balance (in shares) at Dec. 31, 2023	50,632,000	50,632,000
Ending balance at Dec. 31, 2023	$ 255,300	$ 5	$ 0	$ 766,716	$ (204)	$ (511,217)

Consolidated and Combined Sta_4

Consolidated and Combined Statements of Stockholders' Equity (Parenthetical)	Nov. 30, 2021 shares
Pre-funded Warrants \| Private Placement
Pre-funded warrants issued (in shares)	757,575

Consolidated and Combined Sta_5

Consolidated and Combined Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Cash flows from operating activities:
Net loss	$ (217,570)	$ (254,153)	$ (292,213)
Adjustments to reconcile net loss to net cash used in operating activities:
Change in fair value of contingent consideration	235	232	439
Depreciation and amortization	10,285	11,533	16,350
Stock-based compensation expense	32,159	41,040	54,629
Goodwill impairment charge	12,056	0	0
Other non-cash items	(6,627)	85	430
Changes in operating assets and liabilities:
Prepaid expenses and other assets	15,735	(7,332)	1,757
Operating lease right-of-use assets	20,927	34,649	15,541
Accounts payable	(423)	327	(6)
Accrued expenses and other liabilities	(27,713)	(140)	15,733
Operating lease liabilities	(13,499)	(12,042)	(16,199)
Deferred revenue	11,321	(22,762)	4,180
Collaboration research advancement	(3,744)	(19,577)	(7,496)
Net cash used in operating activities	(166,858)	(228,140)	(206,855)
Cash flows from investing activities:
Purchases of property, plant and equipment	(13,865)	(30,810)	(11,575)
Purchases of marketable securities	(246,743)	(145,908)	0
Proceeds from maturities of marketable securities	304,899	180,447	34,986
Purchase of intangible assets	0	0	(8,000)
Purchase of restricted investments	(18,430)	(1,976)	(32,997)
Proceeds from maturities of restricted investments	18,000	2,000	0
Net cash provided by (used in) investing activities	43,861	3,753	(17,586)
Cash flows from financing activities:
Transfers from bluebird bio	0	0	354,889
Proceeds from issuance of common stock in public offering, net of issuance costs	117,004	0	0
Proceeds from issuance of common stock to Regeneron, net of issuance costs	9,859	0	0
Proceeds from issuance of common stock in private placement, net of issuance costs	0	165,533	0
Proceeds from exercise of stock options and ESPP contributions	527	696	0
Net cash provided by financing activities	127,390	166,229	354,889
Increase (decrease) in cash, cash equivalents and restricted cash and cash equivalents	4,393	(58,158)	130,448
Cash, cash equivalents and restricted cash and cash equivalents at beginning of year	72,290	130,448	0
Cash, cash equivalents and restricted cash and cash equivalents at end of year	76,683	72,290	130,448
Reconciliation of cash, cash equivalents and restricted cash and cash equivalents:
Cash and cash equivalents	74,958	71,032	130,414
Restricted cash and cash equivalents included in restricted investments and other non-current assets	1,725	1,258	34
Total cash, cash equivalents and restricted cash and cash equivalents	76,683	72,290	130,448
Supplemental cash flow disclosures:
Purchases of property, plant and equipment included in accounts payable and accrued expenses	855	2,926	3,703
Non-cash return of bRT-related assets to bluebird bio	0	0	110,300
Increase of right-of-use asset and associated lease liability due to lease reassessment	0	0	174,618
Issuance of unrestricted stock awards to settle accrued employee compensation	$ 0	$ 0	$ 6,544

Description of the business

Description of the business	12 Months Ended
Description of the business	Dec. 31, 2023
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Description of the business	Description of the business 2seventy bio, Inc. (the “Company” or “2seventy bio”) is a cell and gene therapy company focused on the research, development, and commercialization of transformative treatments for cancer. The Company’s approach combines its expertise in T cell engineering technology and lentiviral vector gene delivery approaches, experience in research, development, and manufacturing of cell therapies and a suite of technologies that can be selectively deployed to develop highly innovative, targeted cellular therapies for patients with cancer. The Company is advancing multiple preclinical and clinical programs in oncology and, together with BMS, delivering the first FDA-approved CAR T therapy in multiple myeloma, Abecma , to patients in the United States. Please refer to Note 10, Collaborative arrangements and strategic partnerships , for further discussion of the collaboration with BMS. 2seventy bio Securities Corporation is a wholly-owned subsidiary of the Company which was incorporated in Massachusetts on December 13, 2021 and was granted securities corporation status in Massachusetts for the 2021 tax year. 2seventy bio Securities Corporation has no employees. On January 29, 2024, the Company began undertaking a strategic realignment to focus on the development and commercialization of Abecma. In connection with the strategic realignment, the Company entered into an asset purchase agreement with Regeneron Pharmaceuticals, Inc., or Regeneron, to sell to Regeneron substantially all of the assets related to its oncology and autoimmune cell therapy programs . Upon closing of the transaction, which is subject to customary closing conditions, Regeneron will assume all of the ongoing program, infrastructure and personnel costs related to these programs. The transaction is expected to close in the first half of 2024. The Company is subject to risks and uncertainties similar to other companies in the biotechnology industry. There can be no assurance that the Company’s clinical trials will be successfully completed, that it will maintain protection of proprietary technology, or that it will obtain the necessary regulatory approvals for Abecma or any future product candidates. Even following approval, Abecma or any future product candidates may not be commercially viable. The Company operates in an environment of rapid technological innovation and substantial competition from pharmaceutical and biotechnology companies. Additionally, the Company is dependent on key personnel and on third party organizations such as financial institutions, collaborators, and contract manufacturing organizations. The separation from bluebird bio, Inc. In January 2021, bluebird bio, Inc. (“bluebird bio”) announced its plans to separate its oncology portfolio and programs from its severe genetic disease portfolio and programs, and spin off its oncology portfolio and programs into a separate, publicly traded company. In furtherance of this plan, 2seventy bio was incorporated as a Delaware corporation in April 2021, and in September 2021, bluebird bio’s board of directors approved the distribution of all of the issued and outstanding shares of 2seventy bio common stock on the basis of one share of 2seventy bio common stock for every three shares of bluebird bio common stock issued and outstanding on October 19, 2021, the record date for the distribution. As a result of the distribution, which occurred on November 4, 2021, 2seventy bio became an independent, publicly traded company. On November 3, 2021, the Company also entered into a separation agreement with bluebird bio, which is referred to in this annual report as the “Separation Agreement”, as well as various other agreements with bluebird bio, including a tax matters agreement, an employee matters agreement, an intellectual property license agreement, a transition services agreement under which 2seventy bio temporarily receives certain services from bluebird bio, and a second transition services agreement under which 2seventy bio temporarily provides certain services to bluebird bio. These agreements also govern certain of 2seventy bio’s relationships with bluebird bio after the separation. For additional information regarding the Separation Agreement and the other related agreements, refer to Note 14, Related-party transactions. Going concern In accordance with Accounting Standards Codification (“ASC”) 205-40, Going Concern , the Company evaluated whether there are conditions and events, considered in the aggregate, that raise substantial doubt about its ability to continue as a going concern within one year after the date that the consolidated and combined financial statements are issued. The Company has incurred losses and has experienced negative operating cash flows for all historical periods presented. During the year ended December 31, 2023, the Company incurred a net loss of $217.6 million and used $166.9 million of cash in operations. The Company expects to continue to generate operating losses and negative operating cash flows for the near future. As of December 31, 2023, the Company had cash, cash equivalents and marketable securities of $221.8 million. Based on the Company’s current operating plans, including with respect to the ongoing commercialization of Abecma

Summary of significant accounti

Summary of significant accounting policies and basis of presentation	12 Months Ended
	Dec. 31, 2023
Accounting Policies [Abstract]
Summary of significant accounting policies and basis of presentation	Summary of significant accounting policies and basis of presentation Basis of presentation The Company did not operate as a separate, stand-alone entity prior to its separation from bluebird bio. The Company’s consolidated balance sheets as of December 31, 2023 and 2022, statement of operations and comprehensive loss for the years ended December 31, 2023 and 2022, statement of stockholder’s equity for the years ended December 31, 2023 and 2022 and the statement of cash flows for the years ended December 31, 2023 and 2022 consist of the consolidated balances of the Company as prepared on a stand-alone basis. The Company’s consolidated and combined statements of operations and comprehensive loss, stockholders’ equity and cash flows for the year ended December 31, 2021, have been prepared on a carve out basis, derived from bluebird bio’s consolidated financial statements and accounting records, for the period prior to the separation on November 4, 2021 and on a stand-alone basis for the period following the separation through December 31, 2021. The accompanying consolidated and combined financial statements reflect the historical results of the operations, financial position and cash flows of the Company and have been prepared by the Company in accordance with accounting principles generally accepted in the United States (“GAAP”). Any reference in these notes to applicable guidance is meant to refer to the authoritative GAAP as included in the Accounting Standards Codification (“ASC”) and Accounting Standards Updates (“ASUs”) of the Financial Accounting Standards Board (“FASB”). The historical results of operations, financial position and cash flows of 2seventy bio presented in these consolidated and combined financial statements may not be indicative of what they would have been had 2seventy bio been an independent stand-alone entity for each of the years presented prior to the separation, nor are they necessarily indicative of 2seventy bio's future results of operations, financial position and cash flows. Prior to the separation, the Company was dependent upon bluebird bio for all of its working capital and financing requirements, as bluebird bio used a centralized approach to cash management and financing its operations. There were no cash amounts specifically attributable to the Company for the periods prior to the separation; therefore, cash and cash equivalents were not allocated to the Company for periods prior to the separation. Financing transactions related to bluebird bio for periods prior to the separation are accounted for as a component of net parent investment in the consolidated and combined balance sheets and as a financing activity on the accompanying consolidated and combined statements of cash flows. In 2021, net cash transferred to us from bluebird bio of $354.9 million, as indicated in our consolidated and combined statements of cash flows, includes both the net changes in our cash used for operating and investing activities prior to the separation and the cash and cash equivalents distributed to us upon separation. Prior to the separation, the Company’s combined financial statements included an allocation of expenses related to certain bluebird bio corporate functions, including senior management, legal, human resources, finance and information technology. In addition, prior to the separation the Company's combined financial statements include an allocation of certain research and development costs not directly attributable to individual programs. These expenses were allocated to the Company based on direct usage or benefit where specifically identifiable, with the remainder allocated based on employee time spent on projects, square footage or other measures that management believes are consistent and reasonable. These allocations may not be indicative of the actual expense that would have been incurred had the Company operated as an independent, publicly traded company for the periods prior to the separation. See Note 14, Related-party transactions , for a further description of the accounting for the separation from bluebird bio. As 2seventy bio's operations were not historically held by a single legal entity or separate legal entities, net parent investment was shown in lieu of stockholder's equity in the combined financial statements for periods prior to the separation. As a result of the separation, the Company’s net parent investment balance was reclassified to additional paid-in capital. For periods prior to the separation, the taxable income (loss) of bluebird bio entities, including 2seventy bio, Inc. prior to the separation, was included in bluebird bio’s consolidated tax returns. As such, separate income tax returns were not prepared for the entities included within the combined financial statements prior to separation. In connection with the separation, certain assets and liabilities, including certain accounts receivables and accounts payables, included on the consolidated and combined balance sheets prior to the separation have been retained by bluebird bio post-separation and, therefore, were adjusted through net parent investment in the Company’s consolidated and combined financial statements. In addition, in connection with the separation, certain equity awards were converted in accordance with the Employee Matters Agreement, as further described in Note 13, Stock-based compensation . Upon separation, bluebird bio contributed $384.5 million of net assets to the Company. Amounts reported are computed based on thousands, except percentages or as otherwise noted. As a result, certain totals may not sum due to rounding. Correction of immaterial error During the first quarter of 2023, the Company identified two immaterial errors in its previously issued 2022 quarterly reports on Form 10-Q, and 2022 and 2021 annual reports on Form 10-K related to: 1) restricted investments previously presented as restricted cash on its consolidated balance sheets and consolidated statements of cash flows; and 2) cash outflows related to the purchase of property, plant and equipment previously presented within operating cash outflows instead of investing cash outflows in its 2022 annual consolidated statements of cash flows. Based on the analysis of quantitative and qualitative factors in accordance with SEC Staff Accounting Bulletin (SAB) Topic 1.M “Assessing Materiality” and SAB Topic 1.N “Considering the Effects of Prior Year Misstatements when Quantifying Misstatements in Current Year Financial Statements”, the Company concluded that these errors were immaterial, individually and in the aggregate, to its consolidated balance sheets and consolidated statements of cash flows as presented in its previously filed quarterly and annual financial statements. There was no impact to any other statements for any period presented. To correct for the immaterial error related to restricted investments, the Company: • changed the caption “Restricted cash and other non-current assets” to “Restricted investments and other non-current assets” on the balance sheet; • included additional disclosures around the restricted investments within Note 3, Marketable securities and Note 4, Fair value measurements ; and • adjusted its previously filed consolidated statement of cash flows as follows: For the year ended December 31, 2022 in thousands As previously reported Adjustment As revised Cash flows from operating activities: Changes in operating assets and liabilities: Prepaid expenses and other assets $ (8,356) $ 1,024 $ (7,332) Accrued expenses and other liabilities $ (8,110) $ 7,970 $ (140) Net cash used in operating activities $ (237,134) $ 8,994 $ (228,140) Cash flows from investing activities: Purchases of property, plant and equipment $ (22,842) $ (7,968) $ (30,810) Purchases of restricted investments $ — $ (1,976) $ (1,976) Maturities of restricted investments $ — $ 2,000 $ 2,000 Net cash provided by investing activities $ 11,697 $ (7,944) $ 3,753 Decrease in cash, cash equivalents and restricted cash and cash equivalents $ (59,208) $ 1,050 $ (58,158) Cash, cash equivalents and restricted cash and cash equivalents at beginning of period $ 163,266 $ (32,818) $ 130,448 Cash, cash equivalents and restricted cash and cash equivalents at end of period $ 104,058 $ (31,768) $ 72,290 Reconciliation of cash, cash equivalents and restricted cash and cash equivalents Restricted cash and cash equivalents included in restricted investments and other non-current assets $ 33,026 $ (31,768) $ 1,258 Total cash, cash equivalents and restricted cash and cash equivalents $ 104,058 $ (31,768) $ 72,290 For the year ended December 31, 2021 in thousands As previously reported Adjustment As revised Cash flows from operating activities: Changes in operating assets and liabilities: Prepaid expenses and other assets $ 1,578 $ 179 $ 1,757 Net cash used in operating activities $ (207,034) $ 179 $ (206,855) Cash flows from investing activities: Purchases of restricted investments $ — $ (32,997) $ (32,997) Net cash used in (provided by) investing activities $ 15,411 $ (32,997) $ (17,586) Decrease in cash, cash equivalents and restricted cash and cash equivalents $ 163,266 $ (32,818) $ 130,448 Cash, cash equivalents and restricted cash and cash equivalents at end of period $ 163,266 $ (32,818) $ 130,448 Reconciliation of cash, cash equivalents and restricted cash and cash equivalents Restricted cash and cash equivalents included in restricted investments and other non-current assets $ 32,852 $ (32,818) $ 34 Total cash, cash equivalents and restricted cash and cash equivalents $ 163,266 $ (32,818) $ 130,448 The Company has corrected its prior period presentation for this error in the 2023 quarterly financial statements on Form 10-Q and 2023 annual report on Form 10-K. To correct for the immaterial misclassification of cash outflows noted above, the Company has adjusted its 2022 statement of cash flows within its 2023 annual report on Form 10-K by reclassifying $8.0 million of cash outflows from net cash used in operating activities to net cash provided by investing activities. Principles of consolidation and combination Periods prior to separation The accompanying combined financial statements included the attribution of certain assets and liabilities that were historically held by bluebird bio but which were specifically identifiable or attributable to the Company. All intercompany balances and transactions with bluebird bio were deemed to be effectively settled in the combined financial statements at the time the transaction was recorded. Expenses related to corporate allocations from bluebird bio to the Company were considered to be effectively settled for cash in the combined financial statements at the time the transaction was recorded. Periods after the separation The accompanying consolidated and combined financial statements include the accounts of the Company and its wholly owned subsidiaries. All intercompany transactions and accounts within the Company have been eliminated. Certain amounts presented in the prior period have been reclassified to conform to the current period presentation. As a standalone entity, the Company has filed tax returns on its own behalf, and tax balances and the effective income tax rate may have differed from the amounts reported in the historical periods. As of November 4, 2021 and in connection with the separation, the Company adjusted its deferred tax balances and computed its related tax provision to reflect operations as a standalone entity. Variable interest entities The Company continually assesses whether it is the primary beneficiary of a variable interest entity as changes to existing relationships or future transactions may result in consolidation or deconsolidation of one or more collaborators or partners. In determining whether it is the primary beneficiary of an entity in which the Company has a variable interest, management applies a qualitative approach that determines whether the Company has both (1) the power to direct the economically significant activities of the entity and (2) the obligation to absorb losses of, or the right to receive benefits from, the entity that could potentially be significant to that entity. Use of estimates The preparation of financial statements in conformity with GAAP requires management to make estimates and assumptions that affect the reported amounts in the financial statements and accompanying notes. Actual results could materially differ from those estimates. Management considers many factors in selecting appropriate financial accounting policies and controls, and in developing the estimates and assumptions that are used in the preparation of these financial statements. Management must apply significant judgment in this process. In addition, other factors may affect estimates, including: expected business and operational changes, sensitivity and volatility associated with the assumptions used in developing estimates, and whether historical trends are expected to be representative of future trends. The estimation process often may yield a range of potentially reasonable estimates of the ultimate future outcomes and management must select an amount that falls within that range of reasonable estimates. This process may result in actual results differing materially from those estimated amounts used in the preparation of the financial statements. Estimates and judgments are used in the following areas, among others: allocations of revenue, expenses, assets and liabilities from bluebird bio's historical consolidated financial statements to the Company for periods prior to the separation, future undiscounted cash flows and subsequent fair value estimates used to assess potential and measure any impairment of long-lived assets, including goodwill and intangible assets, the measurement of right-of-use assets and lease liabilities, contingent consideration, stock-based compensation expense, accrued expenses, income taxes, and the assessment of the Company's ability to fund its operations for at least the next twelve months from the date of issuance of these financial statements. In addition, estimates and judgments are used in the Company’s accounting for its revenue-generating arrangements, in particular as it relates to determining the stand-alone selling price of performance obligations, evaluating whether an option to acquire additional goods and services represents a material right, estimating the total transaction price, including estimating variable consideration and the probability of achieving future potential development and regulatory milestones, assessing the period of performance over which revenue may be recognized, and accounting for modifications to revenue-generating arrangements. Segment information The Company operates in a single segment, focusing on researching, developing and commercializing potentially transformative treatments for cancer. Consistent with its operational structure, its chief operating decision maker manages and allocates resources for the Company at a consolidated level. Therefore, results of the Company's operations are reported on a consolidated basis for purposes of segment reporting. All material long-lived assets of the Company reside in the United States. Cash and cash equivalents The Company considers all highly liquid investments purchased with original final maturities of 90 days or less from the date of purchase to be cash equivalents. Cash equivalents may consist of marketable securities with maturities of less than 90 days when purchased. Cash equivalents are reported at fair value. There were no cash or cash equivalents specifically attributable to 2seventy bio for the historical periods presented prior to the separation. Restricted investments As of December 31, 2023 and 2022, restricted investments consists primarily of United States government agency securities and treasuries placed as collateral to letters of credit totaling $32.8 million and $30.0 million, respectively that are required to be maintained in connection with the Company’s lease arrangements. The letters of credit are in the name of the Company’s landlord and are required to fulfill lease requirements in the event the Company should default on its lease obligations. As of December 31, 2023 and 2022, the Company classified its restricted investments as non-current on the consolidated balance sheet based on the release dates of the restrictions. For further information on the Company’s letters of credit, see Note 7, Leases . Marketable securities The Company’s marketable securities are maintained by investment managers and consist of U.S. government agency securities and treasuries, corporate bonds and commercial paper. Debt securities are carried at fair value with the unrealized gains and losses included in other comprehensive income (loss) as a component of stockholders’ equity until realized. Any premium arising at purchase is amortized to the earliest call date and any discount arising at purchase is accreted to maturity. Amortization and accretion of premiums and discounts are recorded in interest income, net. Realized gains and losses on debt securities are determined using the specific identification method and are included in other income, net. The Company classifies marketable securities with a remaining maturity when purchased of greater than three months as available-for-sale. Marketable securities with a remaining maturity date greater than one year are classified as non-current assets although these securities are available for use in the Company’s current operations. There were no marketable securities specifically attributable to 2seventy bio for the historical periods presented prior to the separation. The Company assesses its available-for-sale debt securities under the available-for-sale debt security impairment model in ASU 2016-13, Financial Instruments - Credit Losses (Topic 326): Measurement of Credit Losses on Financial Statements (“ASU 2016-13” or “ASC 326”) as of each reporting date in order to determine if a portion of any decline in fair value below carrying value recognized on its available-for-sale debt securities is the result of a credit loss. The Company records credit losses in the consolidated and combined statements of operations and comprehensive loss as credit loss expense within other income, net, which is limited to the difference between the fair value and the amortized cost of the security. To date, the Company has not recorded any credit losses on its available-for-sale debt securities. Accrued interest receivable related to the Company's available-for-sale debt securities is presented within receivables and other current assets on the Company's consolidated balance sheets. The Company has elected the practical expedient available to exclude accrued interest receivable from both the fair value and the amortized cost basis of available-for-sale debt securities for the purposes of identifying and measuring any impairment. The Company writes off accrued interest receivable once it has determined that the asset is not realizable. Any write offs of accrued interest receivable are recorded by reversing interest income, recognizing credit loss expense, or a combination of both. To date, the Company has not written off any accrued interest receivables associated with its marketable securities. Concentrations of credit risk and off-balance sheet risk Financial instruments that subject the Company to credit risk primarily consist of cash and cash equivalents, available-for-sale securities, and accounts receivable. Deposits are maintained with banks in the United States and such amounts may exceed the amount of insurance provided on such deposits. The Company has not experienced any losses on such accounts, and does not believe it is exposed to any unusual credit risk beyond the normal credit risk currently associated with commercial banking relationships. Cash deposits are insured by the Federal Deposit Insurance Corporations (“FDIC”) up to $250,000. The Company has cash deposits in excess of the FDIC insured limit. The Company’s marketable securities, which primarily consist of U.S. government agency securities and treasuries, corporate bonds and commercial paper, potentially subject the Company to concentrations of credit risk. The Company has adopted an investment policy that limits the amounts the Company may invest in any one type of investment and requires all investments held by the Company to be at least AA+/Aa1 rated, thereby reducing credit risk exposure. The Company’s accounts receivable balance primarily consists of amounts owed from collaborative arrangement partners and royalties receivable under out-license agreements with various third parties. The Company monitors economic conditions to identify facts or circumstances that may indicate that any of its accounts receivable are at risk of collection. Fair value of financial instruments The Company has certain financial assets and liabilities recorded at fair value which have been classified as Level 1, 2 or 3 within the fair value hierarchy as described in the accounting standards for fair value measurements: Level 1—Fair values are determined utilizing quoted prices (unadjusted) in active markets for identical assets or liabilities that the Company has the ability to access at the measurement date. Level 2—Fair values are determined utilizing quoted prices for identical assets or liabilities in non-active markets, quoted prices for similar assets or liabilities in active markets, or other market observable inputs such as interest rates, yield curves and foreign currency spot rates. Level 3—Prices or valuations that require inputs that are both significant to the fair value measurement and unobservable. To the extent that valuation is based on models or inputs that are less observable or unobservable in the market, the determination of fair value requires more judgment. Accordingly, the degree of judgment exercised by the Company in determining fair value is greatest for instruments categorized in Level 3. A financial instrument’s level within the fair value hierarchy is based on the lowest level of any input that is significant to the fair value measurement. Items measured at fair value on a recurring basis include marketable securities (see Note 3, Marketable securities , and Note 4, Fair value measurements ) and restricted cash and cash equivalents, restricted investments, and contingent consideration (see Note 4, Fair value measurements ). The carrying amounts of accounts receivable, accounts payable and accrued expenses approximate their fair values due to their short-term nature. Business combinations Business combinations are accounted for using the acquisition method of accounting. Using this method, the tangible and intangible assets acquired and the liabilities assumed are recorded as of the acquisition date at their respective fair values. The Company evaluates a business as an integrated set of activities and assets that is capable of being managed for the purpose of providing a return in the form of dividends, lower costs or other economic benefits and consists of inputs and processes that provide or have the ability to provide outputs. In an acquisition of a business, the excess of the fair value of the consideration transferred over the fair value of the net assets acquired is recorded as goodwill. In an acquisition of net assets that does not constitute a business, no goodwill is recognized. The consolidated and combined financial statements include the results of operations of an acquired business after the completion of the acquisition. Goodwill Goodwill represents the excess of the purchase price over the fair value of the net assets acquired when accounted for using the acquisition method of accounting. Goodwill is not amortized; rather, it is evaluated for impairment within the Company’s single reporting unit on an annual basis, during the fourth quarter, or more frequently if an event occurs or circumstances change that would more-likely-than-not reduce the fair value of the Company’s reporting unit below its carrying amount. The Company performs a one-step quantitative test and records the amount of goodwill impairment, if any, as the excess of a reporting unit's carrying amount over its fair value, not to exceed the total amount of goodwill allocated to the reporting unit. As noted within “Change in Fair Value of Contingent Consideration” above, the Company assumed goodwill related to the Pregenen acquisition upon its separation from bluebird bio. As discussed further in Note 17, Goodwill , the sustained decline in the price of the Company’s common stock in part due to decreased external expectations for future Abecma sales resulting from increased competitive dynamics triggered an indicator of impairment during the third quarter of 2023. Upon completing a quantitative goodwill impairment test, the Company recorded a non-cash impairment charge of $12.1 million, writing off its goodwill balance in it’s entirety. Intangible assets, net Intangible assets, net primarily consist of acquired core technology, net of accumulated amortization. The Company amortizes its intangible assets using the straight-line method over their estimated economic lives and periodically reviews for impairment. The Company has not recognized any impairment charges related to intangible assets to date. Contingent consideration Each reporting period, the Company remeasures the contingent consideration obligations associated with business combinations to their fair value and records within operating expenses increases or decreases in their fair value as change in fair value of contingent consideration within the consolidated and combined statements of operations and comprehensive loss. Changes in contingent consideration result from changes in the assumptions regarding probabilities of successful achievement of related milestones, the estimated timing in which the milestones may be achieved, and the discount rate used to estimate the fair value of the liability. Contingent consideration may change significantly as development of the Company’s programs in certain indications progress and additional data is obtained, impacting the Company’s assumptions. The assumptions used in estimating fair value require significant judgment. The use of different assumptions and judgments could result in a materially different estimate of fair value. See Note 4, Fair value measurements, for additional information. Property, plant and equipment Property, plant and equipment is stated at cost. Maintenance and repairs that do not improve or extend the lives of the respective assets are expensed to operations as incurred. Depreciation and amortization is calculated using the straight-line method over the estimated useful lives of the assets, which are as follows: Asset Estimated useful life Building 40 years Computer equipment and software 3 years Furniture and fixtures 2-5 years Laboratory equipment 5-10 years Leasehold improvements Shorter of the useful life or remaining lease term Impairment of long-lived assets The Company reviews long-lived assets when events or changes in circumstances indicate the carrying value of the assets may not be recoverable. Recoverability is measured by comparison of the book values of the assets to future net undiscounted cash flows that the assets are expected to generate. If such assets are considered to be impaired, the impairment to be recognized is measured by the amount by which the book value of the assets exceed their fair value, which is measured based on the projected discounted future net cash flows arising from the assets. Leases At the inception of an arrangement, the Company determines whether the arrangement is or contains a lease based on the relevant facts and circumstances present in the arrangement. Leases with a term greater than one year are recognized on the balance sheet as right-of-use assets and current and non-current lease liabilities, as applicable. The Company does not have material financing leases. Operating lease liabilities and their corresponding right-of-use assets are initially recorded based on the present value of lease payments over the expected remaining lease term. Certain adjustments to the right-of-use asset may be required for items such as incentives received. The interest rate implicit in lease contracts is typically not readily determinable. As a result, the Company utilizes its incremental borrowing rate to discount lease payments, which reflects the rate at which the Company could borrow on a collateralized basis the amount of the lease payments in the same currency, for a similar term, in a similar economic environment. To estimate its incremental borrowing rate, a credit rating applicable to the Company is estimated using a synthetic credit rating analysis since the Company does not currently have a rating agency-based credit rating. The Company has elected not to recognize leases with an original term of one year or less on the balance sheet. The Company typically only includes an initial lease term in its assessment of a lease arrangement. Options to renew a lease are not included in the Company’s assessment unless it is reasonably certain that the Company will exercise its renewal option. Assumptions made by the Company at the commencement date are re-evaluated upon occurrence of certain events, including a lease modification. A lease modification results in a separate contract when the modification grants the lessee an additional right of use not included in the original lease and when lease payments increase commensurate with the stand-alone price for the additional right of use. When a lease modification results in a separate contract, it is accounted for in the same manner as a new lease. In accordance with ASC 842, components of a lease should be separated into lease components and non-lease components. The fixed and in-substance fixed contract consideration must be allocated based on the relative stand-alone prices to the lease components and non-lease components. Entities may elect not to separate lease and non-lease components. Rather, entities would account for each lease component and related non-lease component together as a single lease component. The Company has elected to account for lease and non-lease components together as a single lease component for all underlying assets and allocate all of the contract consideration to the lease component only. For purposes of distinguishing between operating and financing leases, ASC 842 allows for the use of judgment in determining whether the lease term is for a major part of the remaining economic life of the underlying asset and whether the present value of lease payments represents substantially all of the fair value of the underlying asset. The Company applies the bright line thresholds referenced in ASC 842-10-55-2 to assist in evaluating leases for appropriate classification. The aforementioned bright lines are applied consistently to the Company’s entire portfolio of leases. Common stock warrants The Company's common stock warrants are evaluated pursuant to ASC 480, Distinguishing Liabilities from Equity ("ASC 480"), and ASC 815, Derivatives and Hedging ("ASC 815"). Management classifies its freestanding warrants as (i) liabilities, if the warrant terms allow settlement of the warrant exercise in cash, or (ii) equity, if the warrant terms only allow settlement in shares of common stock. Revenue recognition Under ASC Topic 606, Revenue from Contracts with Customers (“Topic 606”), an entity recognizes revenue when its customer obtains control of promised goods or services, in an amount that reflects the consideration that the entity expects to receive in exchange for those goods or services. To determine revenue recognition for arrangements that an entity determines are within the scope of Topic 606, the entity performs the following five steps: (i) identify the contract(s) with a customer; (ii) identify the performance obligations in the contract; (iii) determine the transaction price, including variable consideration, if any; (iv) allocate the transaction price to the performance obligations in the contract; and (v) recognize revenue when (or as) the entity satisfies a performance obligation. The Company only applies the five-step model to contracts when it is probable that the entity will collect the consideration to which it is entitled in exchange for the goods or services it transfers to the customer. Once a contract is determined to be within

Marketable securities

Marketable securities	12 Months Ended
Marketable securities	Dec. 31, 2023
Investments, Debt and Equity Securities [Abstract]
Marketable securities	Marketable securities The following table summarizes the marketable securities held at December 31, 2023 and December 31, 2022 (in thousands): Amortized Unrealized gains Unrealized losses Fair Value December 31, 2023 U.S. government agency securities and $ 101,566 $ 144 $ (85) $ 101,625 Commercial paper 45,188 34 — 45,222 Total $ 146,754 $ 178 $ (85) $ 146,847 December 31, 2022 U.S. government agency securities and $ 120,739 $ 3 $ (1,963) $ 118,779 Corporate bonds 2,524 — (26) 2,498 Commercial paper 75,491 3 (119) 75,375 Total $ 198,754 $ 6 $ (2,108) $ 196,652 No available-for-sale debt securities held as of December 31, 2023 or December 31, 2022 had remaining maturities greater than five years. The following table summarizes available-for-sale debt securities in a continuous unrealized loss position for less than and greater than twelve months, and for which an allowance for credit losses has not been recorded at December 31, 2023 and December 31, 2022 (in thousands): Less than 12 months 12 months or greater Total Description Fair value Unrealized losses Fair value Unrealized losses Fair value Unrealized losses December 31, 2023 U.S. government agency securities and $ 45,850 $ (60) $ 1,475 $ (25) $ 47,325 $ (85) Total $ 45,850 $ (60) $ 1,475 $ (25) $ 47,325 $ (85) December 31, 2022 U.S. government agency securities and $ 28,749 $ (159) $ 86,176 $ (1,804) $ 114,925 $ (1,963) Corporate bonds — — 2,498 (26) 2,498 (26) Commercial paper 62,636 (119) — — 62,636 (119) Total $ 91,385 $ (278) $ 88,674 $ (1,830) $ 180,059 $ (2,108) As discussed further in Note 7, Leases , the Company maintains letters of credit related to its leases in Cambridge and Seattle. A portion of this collateral is classified as restricted investments and included within restricted investments and other non-current assets on the condensed consolidated balance sheets. The following table summarizes restricted investments held at December 31, 2023 and December 31, 2022 (in thousands): Amortized Unrealized gains Unrealized losses Fair Value December 31, 2023 U.S. government agency securities and $ 33,072 $ 67 $ (365) $ 32,774 Total $ 33,072 $ 67 $ (365) $ 32,774 December 31, 2022 U.S. government agency securities and $ 32,880 $ — $ (1,112) $ 31,768 Total $ 32,880 $ — $ (1,112) $ 31,768 The following table summarizes restricted investments in a continuous unrealized loss position for less than and greater than twelve months, and for which an allowance for credit losses has not been recorded at December 31, 2023 and December 31, 2022 (in thousands): Less than 12 months 12 months or greater Total Description Fair value Unrealized losses Fair value Unrealized losses Fair value Unrealized losses December 31, 2023 U.S. government agency securities and $ 3,496 $ (4) $ 13,266 $ (361) $ 16,762 $ (365) Total $ 3,496 $ (4) $ 13,266 $ (361) $ 16,762 $ (365) December 31, 2022 U.S. government agency securities and $ 1,942 $ (27) $ 29,826 $ (1,085) $ 31,768 $ (1,112) Total $ 1,942 $ (27) $ 29,826 $ (1,085) $ 31,768 $ (1,112) Accrued interest receivable on the Company's available-for-sale debt securities totaled $0.8 million and $0.3 million as of December 31, 2023 and 2022, respectively. No accrued interest receivable was written off during the twelve months ended December 31, 2023 or 2022. The amortized cost of available-for-sale debt securities is adjusted for amortization of premiums and accretion of discounts to the earliest call date for premiums or to maturity for discounts. At December 31, 2023 and December 31, 2022, the balance in the Company’s accumulated other comprehensive loss was composed primarily of activity related to the Company’s available-for-sale debt securities. There were no material realized gains or losses recognized on the sale or maturity of available-for-sale debt securities during the years ended December 31, 2023 or December 31, 2022. The Company determined that there was no material change in the credit risk of the above investments for the year ended December 31, 2023. As such, an allowance for credit losses was not recognized. As of December 31, 2023, the Company does not intend to sell such securities and it is not more likely than not that the Company will be required to sell the securities before recovery of their amortized cost bases.

Fair value measurements

Fair value measurements	12 Months Ended
Fair value measurements	Dec. 31, 2023
Fair Value Disclosures [Abstract]
Fair value measurements	Fair value measurements The following table sets forth the Company’s assets and liabilities that are measured at fair value on a recurring basis as of December 31, 2023 and 2022 (in thousands): Total Quoted prices in active markets (Level 1) Significant other observable inputs (Level 2) Significant unobservable inputs (Level 3) December 31, 2023 Assets: Cash and cash equivalents $ 74,958 $ 74,958 $ — $ — Marketable securities: U.S. government agency securities and treasuries 101,625 — 101,625 — Commercial paper 45,222 — 45,222 — Restricted cash and cash equivalents 1,725 1,725 — Restricted investments 32,774 — 32,774 — Total assets $ 256,304 $ 76,683 $ 179,621 $ — Liabilities: Contingent consideration $ 2,415 $ — $ — $ 2,415 Total liabilities $ 2,415 $ — $ — $ 2,415 December 31, 2022 Assets: Cash and cash equivalents $ 71,032 $ 71,032 $ — $ — Marketable securities: U.S. government agency securities and treasuries 118,779 — 118,779 — Corporate bonds 2,498 — 2,498 — Commercial paper 75,375 — 75,375 — Restricted cash and cash equivalents 1,258 1,258 — — Restricted investments 31,768 — 31,768 — Total assets $ 300,710 $ 72,290 $ 228,420 $ — Liabilities: Contingent consideration $ 2,180 $ — $ — $ 2,180 Total liabilities $ 2,180 $ — $ — $ 2,180 Marketable securities Marketable securities classified as Level 2 within the valuation hierarchy generally consist of U.S. government agency securities and treasuries, corporate bonds, and commercial paper. The Company estimates the fair values of these marketable securities by taking into consideration valuations obtained from third-party pricing sources. These pricing sources utilize industry standard valuation models, including both income and market-based approaches, for which all significant inputs are observable, either directly or indirectly, to estimate fair value. These inputs include market pricing based on real-time trade data for the same or similar securities, issuer credit spreads, benchmark yields, and other observable inputs. The Company validates the prices provided by its third-party pricing sources by understanding the models used, obtaining market values from other pricing sources and analyzing pricing data in certain instances. Contingent consideration In connection with bluebird bio's prior acquisition of Precision Genome Engineering, Inc. (“Pregenen”), the Company may be required to pay future consideration that is contingent upon the achievement of specified development, regulatory approvals or sales-based milestone events. Contingent consideration is measured at fair value and is based on significant unobservable inputs, which represents a Level 3 measurement within the fair value hierarchy. The valuation of contingent consideration uses assumptions the Company believes would be made by a market participant. The Company assesses these estimates on an on-going basis as additional data impacting the assumptions is obtained. Future changes in the fair value of contingent consideration related to updated assumptions and estimates are recognized within the consolidated and combined statements of operations and comprehensive loss. In the absence of new information related to the probability of milestone achievement, changes in fair value will reflect changing discount rates and the passage of time. Contingent consideration is included in other non-current liabilities on the consolidated balance sheets. The table below provides a roll-forward of fair value of the Company’s contingent consideration obligations that include Level 3 inputs (in thousands): Year ended December 31, 2023 2022 Beginning balance $ 2,180 $ 1,948 Additions — — Changes in fair value 235 232 Payments — — Ending balance $ 2,415 $ 2,180 Please refer to Note 8, Commitments and contingencies, for further information.

Property, plant and equipment,

Property, plant and equipment, net	12 Months Ended
Property, plant and equipment, net	Dec. 31, 2023
Property, Plant and Equipment [Abstract]
Property, plant and equipment, net	Property, plant and equipment, net Property, plant and equipment, net, consists of the following (in thousands): As of December 31, 2023 2022 Computer equipment and software 6,156 5,670 Office equipment 6,726 6,159 Laboratory equipment 43,209 36,216 Leasehold improvements 58,832 27,416 Construction-in-progress 138 28,112 Total property, plant and equipment 115,061 103,573 Less accumulated depreciation and amortization (56,911) (47,838) Property, plant and equipment, net $ 58,150 $ 55,735 Depreciation and amortization expense related to property, plant and equipment was $9.6 million, $8.9 million, and $12.1 million for the years ended December 31, 2023, 2022, and 2021, respectively. Cambridge, Massachusetts drug product manufacturing facility In February 2022, the Company began construction of a drug product manufacturing facility within its Cambridge, Massachusetts headquarters. The facility enables rapid translational research in clinical trials and the manufacture of drug product for use in Phase 1 clinical development activities. The facility became fully operational as of August 2023 and related c onstruction-in-progress totaling $36.6 million related to the build-out of the facility was placed into service. As of December 31, 2022, construction-in-progress included $27.0 million related to the build-out of the facility.

Accrued expenses and other curr

Accrued expenses and other current liabilities	12 Months Ended
Accrued expenses and other current liabilities	Dec. 31, 2023
Payables and Accruals [Abstract]
Accrued expenses and other current liabilities	Accrued expenses and other current liabilities Accrued expenses and other current liabilities consist of the following (in thousands): As of December 31, 2023 2022 Royalties $ 9,702 $ 13,094 Collaboration research costs 5,681 2,005 Employee compensation 4,639 14,845 Manufacturing costs 1,764 17,962 Clinical and contract research organization costs 990 1,619 Property, plant, and equipment 279 1,498 Other 2,633 3,655 Total accrued expenses and other current liabilities $ 25,688 $ 54,678 The decrease in accrued expenses as of December 31, 2023 compared to December 31, 2022 is largely due to a decrease in accrued manufacturing costs. During 2022, the Company accrued $14.8 million representing its estimated share of the net operating losses of and relating to the manufacturing facility under the assignments pertinent to the asset purchase agreement and the Development Manufacturing Supply Agreement committing the Company to reimburse Resilience. The Company settled this amount with Resilience in early 2023. Refer to Note 8, Commitments & Contingencies. Accrued employee compensation expenses also significantly decreased as of December 31, 2023 compared to December 31, 2022 primarily due to the payout of 2023 annual bonuses in December 2023, whereas 2022 annual bonuses were accrued as of December 31, 2022 and paid out in the first quarter of 2023. Accrued royalties decreased as of December 31, 2023 compared to December 31, 2022 due to a decline in Abecma sales in the fourth quarter of 2023 compared to the fourth quarter of 2022.

Leases

Leases	12 Months Ended
Leases	Dec. 31, 2023
Leases [Abstract]
Leases	Leases The Company leases certain office and laboratory space that was attributed to it in connection with the separation. 60 Binney Street lease In September, 2015, bluebird bio entered into a lease agreement, which was attributed to the Company in connection with the separation. This is the Company’s corporate headquarters and includes office and laboratory space located in a building (the “Building”) at 60 Binney Street, Cambridge, Massachusetts (the “60 Binney Street Lease”). Under the terms of the 60 Binney Street Lease, starting on October 1, 2016, bluebird bio leased approximately 253,108 square feet of office and laboratory space at $72.50 per square foot per year, or $18.4 million per year in base rent, which is subject to scheduled annual rent increases of 1.75% plus certain operating expenses and taxes. bluebird bio historically maintained a $13.8 million collateralized letter of credit which, subject to the terms of the lease and certain reduction requirements specified therein, including market capitalization requirements, may decrease to $9.2 million over time. As the Company did not have legal ownership over any bank accounts prior to the separation, there were no cash and cash equivalent balances specifically attributable to the Company for the periods prior to the separation and, accordingly, no restricted cash is reflected in the consolidated and combined financial statements related to the letter of credit for periods prior to the separation. Pursuant to a work letter entered into in connection with the 60 Binney Street Lease, the landlord contributed an aggregate of $42.4 million toward the cost of construction and tenant improvements for the Building. The lease would continue until March 31, 2027. In October 2021, bluebird bio entered into a consent to assignment and amendment to its lease agreement for its 60 Binney Street lease, pursuant to which bluebird bio's interest in the lease was assigned to the Company. In November 2021, the Company executed a $25.0 million letter of credit related to this lease. Under the assigned and amended lease agreement, the lease term was extended through March 31, 2034 with the 2022 base rent being $90.00 per square foot per year, or $22.8 million per year in base rent, subject to scheduled annual rent increases of 3.0% plus certain operating expenses and taxes. Starting April 1, 2026, the base rent will reset to $118.41 per square foot per year, or $30.0 million per year in base rent, subject to scheduled annual rent increases of 3.0% plus certain operating expenses and taxes. Pursuant to a work letter entered into in connection with the 60 Binney Street assignment and amended lease agreement, the landlord agreed to contribute up to an aggregate of $19.1 million toward the cost of tenant improvements for the leased space. As of December 31, 2023, the Company has spent $36.6 million related to the build-out of the drug product manufacturing facility within its Cambridge, Massachusetts headquarters and has received $18.7 million in reimbursements. The Company has classified this lease as an operating lease and recorded a right-of-use asset and lease liability. The Company is recognizing rent expense on a straight-line basis throughout the remaining term of the lease. Seattle, Washington leases In July 2018, bluebird bio entered into a lease agreement for office and laboratory space located in a portion of a building in Seattle, Washington, and moved into the facility in June 2019. This lease was assigned to the Company in connection with the separation. The lease was amended in October 2018 to increase the total rentable space to approximately 36,126 square feet at $54.00 per square foot in base rent per year, which is subject to scheduled annual rent increases of 2.5% plus certain operating expenses and taxes. The lease commenced on January 1, 2019 and the lease term will continue through January 31, 2027. The Company has classified this lease as an operating lease and recorded a right-of-use asset and lease liability at lease commencement. In September 2019, bluebird bio entered into a second amendment to the lease (the “Second Amendment”). The Second Amendment added approximately 22,188 square feet to the existing space and extended the lease term of the entire premises by 16 months, or until April 2028. Fixed monthly rent for the expanded space will be incurred at a rate of $62.80 per square foot per year beginning in January 2021, subject to annual increases of 2.5%. The Second Amendment includes a five-year option to extend the term. The execution of the Second Amendment was deemed to be a lease modification representing two separate contracts under ASC 842. One contract is related to a new right-of-use for the expanded 22,188 square feet of space, which is to be accounted for as a new lease, and the other is related to the modification of term for the original 36,126 square feet of space. An additional right-of-use asset and lease liability was recognized upon lease commencement of the expanded space. In September 2020, bluebird bio entered into a sublease agreement for the 22,188 square feet added under the Second Amendment at a fixed monthly rent of $62.80 per square foot per year beginning in January 2021, subject to annual increases of 2.5%. The sublease term will continue through April 2028. In October 2021, bluebird bio entered into a consent to assignment and amendment to its lease agreement for office and laboratory space in Seattle, Washington and the related sublease that was executed in September 2020 for a portion of the space. The agreement reassigns bluebird bio’s interest in the lease and the sublease to the Company. In November 2021, the Company executed a $5.0 million letter of credit related to this lease. The Company is recognizing rent expense on a straight-line basis through the remaining extended term of the respective leases. The head lease and the sublease will be accounted for as two separate contracts with the income from the sublease presented separately from the lease expense on the head lease. Summary of all lease costs recognized under ASC 842 The following table contains a summary of the lease costs recognized under ASC 842 and other information pertaining to the Company’s operating leases for the years ended December 31, 2023, 2022 and 2021 (in thousands): For the year ended December 31, 2023 2022 2021 Lease cost (1) Operating lease cost $ 32,636 $ 33,738 $ 24,838 Total lease cost $ 32,636 $ 33,738 $ 24,838 Other information Operating cash flows used for operating leases $ 27,927 $ 27,140 $ 25,489 Weighted average remaining lease term 9.9 years 10.8 years 11.8 years Weighted average discount rate 5.48 % 5.47 % 5.47 % ________________ (1) Short-term lease costs incurred by the Company for the twelve months ended December 31, 2023, 2022 and 2021 were immaterial. Rent expense is calculated on a straight-line basis over the term of the lease. Rent expense recognized under all leases, including additional variable costs for utilities, parking, maintenance, and real estate taxes that are not included within lease costs in the table above, was $43.4 million, $43.9 million, and $34.5 million for the years ended December 31, 2023, 2022 and 2021, respectively. As of December 31, 2023, future minimum commitments under ASC 842 under the Company’s operating leases were as follows (in thousands): Maturity of lease liabilities As of December 31, 2023 2024 $ 28,672 2025 29,498 2026 32,920 2027 35,086 2028 and thereafter 215,296 Total lease payments 341,472 Less: imputed interest (84,799) Total operating lease liabilities $ 256,673 Please refer to Note 20, Subsequent Events , for further information on the terms of the Asset Sale to Regeneron. In connection with the Asset Sale,

Commitments and contingencies

Commitments and contingencies	12 Months Ended
Commitments and contingencies	Dec. 31, 2023
Commitments and Contingencies Disclosure [Abstract]
Commitments and contingencies	Commitments and contingencies Lease commitments 2seventy bio leases certain office and laboratory space. Refer to Note 7, Leases , for further information on the terms of these lease agreements. Contingent consideration related to business combinations On June 30, 2014, bluebird bio acquired Pregenen. All assets and liabilities related to the Pregenen acquisition, including the resulting goodwill and contingent consideration, were attributed to the Company in connection with the separation. The Company may be required to make up to an additional $99.9 million in remaining future contingent cash payments to the former equityholders of Pregenen upon the achievement of certain commercial milestones related to the Pregenen technology. In accordance with accounting guidance for business combinations, contingent consideration liabilities are required to be recognized on the consolidated balance sheets at fair value. Estimating the fair value of contingent consideration requires the use of significant assumptions primarily relating to probabilities of successful achievement of certain clinical and commercial milestones, the expected timing in which these milestones will be achieved and discount rates. The use of different assumptions could result in materially different estimates of fair value. Other funding commitments Certain agreements that were assigned by bluebird bio to the Company in connection with the separation relate principally to licensed technology and may require future payments relating to milestones that may be met in subsequent periods or royalties on future sales of specified products. These agreements include the collaboration agreements entered into with BMS, Regeneron, Novo Nordisk and the agreements entered into with Resilience, all of which were assigned to the Company in connection with the separation. Additionally, to the extent an agreement relating to licensed technology was not attributed to the Company, bluebird bio entered into a sublicense with the Company, which may require future milestone and/or royalty payments. Please refer to Note 10, Collaborative arrangements and strategic partnerships , for further information on the BMS, Regeneron, and Resilience agreements and Note 11, Royalty and other revenue , for further information on license agreements. Based on the Company's development plans as of December 31, 2023, the Company may be obligated to make future development, regulatory and commercial milestone payments and royalty payments on future sales of specified products. Payments under these agreements generally become due and payable upon achievement of such milestones or sales. When the achievement of these milestones or sales has not occurred, such contingencies are not recorded in the Company’s financial statements. As further discussed in Note 10, Collaborative arrangements and strategic partnerships , BMS assumed responsibility for amounts due to licensors as a result of any future ex-U.S. sales of Abecma . In July 2021, bluebird bio and National Resilience, Inc. (“Resilience”) announced a strategic manufacturing collaboration aimed to accelerate the early research, development, and delivery of cell therapies. Agreements related to the collaboration were executed in September 2021. As part of the agreement, Resilience acquired bluebird bio's North Carolina manufacturing facility and retained all staff employed at the site. Concurrent with the sale of the manufacturing facility in Durham, North Carolina, bluebird bio entered into certain ancillary agreements, including two manufacturing agreements and a license agreement (the “Resilience License Agreement”), among others (together referred to as the “Ancillary Agreements”). One of the manufacturing agreements supports ongoing manufacturing for lentiviral vector for development candidates (the “Development Manufacturing Supply Agreement”). The other manufacturing agreement for the future manufacturing of lentiviral vector for the commercial product marketed in collaboration with BMS, Abecma (the “Commercial Supply Agreement”) was assigned by the Company to BMS on June 23, 2023. Certain rights and obligations under the Ancillary Agreements were assigned by bluebird bio to 2seventy bio on November 4, 2021 upon the separation of 2seventy bio from bluebird bio. The assignments under the asset purchase agreement and the Development Manufacturing Supply Agreement commit the Company to reimburse Resilience for an amount equal to 50% of the net operating losses of and relating to the manufacturing facility’s business incurred during the twelve-month period ending on the first anniversary of the closing of the transaction, as calculated in accordance with the asset purchase agreement, subject to a cap of $15.0 million. During the second quarter of 2023, the Company paid a total of $14.2 million to Resilience for its share of net operating losses. The disposition of the net assets of the manufacturing facility previously assigned to 2seventy bio was reflected as a transfer to bluebird bio via net parent investment as a result of bluebird bio’s sale of such facility in the Company’s 2021 annual report on Form 10-K. As a result of the separation, the Company’s net parent investment balance was reclassified to additional paid-in capital. 2seventy bio is not a party to the sale of the manufacturing facility and, therefore, did not recognize any gain or loss arising from the transaction. The Company has various manufacturing development and license agreements to support clinical and commercial product needs. The following table presents non-cancelable contractual obligations arising from these arrangements: Years ended December 31, Purchase commitment 2024 $ 6,671 2025 and thereafter — Total purchase commitments $ 6,671 Litigation From time to time, the Company expects to be party to various claims and complaints arising in the ordinary course of business. However, the Company is not currently a party to any litigation or legal proceedings that, in the opinion of its management, are probable of having a material adverse effect on its business. The Company enters into standard indemnification agreements in the ordinary course of business. Pursuant to these agreements, the Company indemnifies, holds harmless, and agrees to reimburse the indemnified party for losses suffered or incurred by the indemnified party, generally the Company’s business partners. In addition, pursuant to the separation agreement, the Company indemnifies, holds harmless, and agrees to reimburse bluebird bio for its indemnification obligations with respect to the Company’s business partners, relating to the Company’s business or arising out of the Company’s activities, in the past or to be conducted in the future. The term of these indemnification agreements is generally perpetual any time after execution of the agreement. The maximum potential amount of future payments the Company could be required to make under these indemnification agreements is unlimited. Management does not believe that any ultimate liability resulting from any of these claims will have a material adverse effect on its results of operations, financial position, or liquidity. However, management cannot give any assurance regarding the ultimate outcome of any claims, and their resolution could be material to operating results for any particular period. The Company indemnifies each of its directors and officers for certain events or occurrences, subject to certain limits, while the officer or director is or was serving at the Company’s request in such capacity, as permitted under Delaware law and in accordance with its certificate of incorporation and by-laws. The term of the indemnification period will last as long as a director or officer may be subject to any proceeding arising out of acts or omissions of such director or officer in such capacity. The maximum amount of potential future indemnification is unlimited; however, the Company holds director and officer liability insurance.

Stockholders' equity

Stockholders' equity	12 Months Ended
Stockholders' equity	Dec. 31, 2023
Equity [Abstract]
Stockholders' equity	Stockholders’ equity The Company is authorized to issue 200.0 million shares of common stock. Holders of common stock are entitled to one vote per share. Holders of common stock are entitled to receive dividends, if and when declared by the Company’s board of directors, and to share ratably in the Company’s assets legally available for distribution to the Company’s shareholders in the event of liquidation. Holders of common stock have no preemptive, subscription, redemption or conversion rights. As of December 31, 2023, the Company had 50.6 million shares of common stock issued and outstanding. As of December 31, 2022, the Company had 37.9 million shares of common stock issued and outstanding. In November 2021, the Company issued 23,368,988 shares of its common stock to bluebird bio in connection with the separation. In November 2021, the Company issued to certain institutional investors (who previously purchased pre-funded warrants to purchase shares of bluebird bio common stock) pre-funded warrants to purchase 757,575 shares of the Company’s common stock at an exercise price of $0.0001 per share. The pre-funded warrants can be exercised at any time or times on or after November 4, 2021, until exercised in full. The warrants have been evaluated to determine the appropriate accounting and classification pursuant to ASC 480 and ASC 815. Based on the terms of the pre-funded warrants, management concluded that they should be classified within stockholders’ equity on the Company’s consolidated balance sheets, with no subsequent remeasurement as long as the underlying warrant agreements are not modified or amended. In March 2022, the Company entered into stock purchase agreements with certain investors, pursuant to which the Company agreed to sell and issue, in a private placement, an aggregate of 13,934,427 shares of the Company’s common stock at a purchase price per share of $12.20. This resulted in aggregate net proceeds to the Company of $165.5 million, after deducting placement agent fees and other offering expenses payable by the Company. In November 2022, the Company entered into a sales agreement with Cowen and Company, LLC (“Cowen”) under which it may from time to time offer and sell shares of its common stock through Cowen for aggregate gross sales proceeds of up to $150.0 million (the “ATM facility”). The Company has not sold any shares of its common stock under the ATM Facility as of the date of this Annual Report on Form 10-K and the Company does not currently have any plans to sell shares under the ATM. In January 2023, the Company entered into a Share Purchase Agreement with Regeneron, pursuant to which it sold 1,114,827 shares of its common stock to Regeneron, subject to certain restrictions, for an aggregate cash price of approximately $20.0 million. The purchase price represents $9.9 million worth of common stock plus a $10.1 million premium, which represents collaboration deferred revenue. Details regarding the recognition of this deferred revenue as revenue are included below in Note 10, Collaborative arrangements and strategic partnerships. In March 2023, the Company sold 10,869,566 shares of common stock through an underwritten public offering at a price per share of $11.50. This resulted in aggregate net proceeds to the Company of approximately $117.0 million, after deducting underwriting fees and offering expenses. The underwriters did not exercise their option to purchase up to 1,630,434 additional shares of common stock and therefore no additional proceeds were received. The Company is authorized to issue 10.0 million shares of preferred stock in one or more series and to fix the powers, designations, preferences and relative participating option or other rights thereof, including dividend rights, conversion rights, voting rights, redemption terms, liquidation preferences and the number of shares constituting any series, without any further vote or action by the Company’s shareholders. As of December 31, 2023 and 2022, the Company had no shares of preferred stock issued or outstanding. Reserved for future issuance The Company has reserved for future issuance the following number of shares of common stock (in thousands): As of December 31, 2023 2022 Options to purchase common stock (1) 3,678 2,562 Restricted stock units (1) 2,239 1,314 2021 Stock Option and Incentive Plan 871 1,621 2021 Employee Stock Purchase Plan 552 434 Pre-funded warrants to purchase common stock 758 758 8,098 6,689 (1)

Collaborative arrangements and

Collaborative arrangements and strategic partnerships	12 Months Ended
Collaborative arrangements and strategic partnerships	Dec. 31, 2023
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Collaborative arrangements and strategic partnerships	Collaborative arrangements and strategic partnerships To date, the Company’s service and collaborative arrangement revenue has been primarily generated from collaboration arrangements with BMS, Regeneron, and Novo, each as further described below. Bristol Myers Squibb BMS Collaboration Agreement In March 2013, bluebird bio entered into a Master Collaboration Agreement (the “BMS Collaboration Agreement”) with Celgene (now BMS following its acquisition of Celgene in November 2019) to discover, develop and commercialize potentially disease-altering gene therapies in oncology and a Platform Technology Sublicense Agreement (the “Sublicense Agreement”) with BMS pursuant to which bluebird bio obtained a sublicense to certain intellectual property from BMS, originating under BMS’s license from Baylor College of Medicine, for use in the collaboration. Under the terms of the BMS Collaboration Agreement, the Company received an up-front, non-refundable, non-creditable payment of $75.0 million. The Company was responsible for conducting discovery, research, and development activities through completion of Phase 1 clinical trials, if any, during the initial term of the BMS Collaboration Agreement, or three years. Subsequently, bluebird bio and BMS executed various amendments, summarized as follows. These agreements were assumed by the Company in connection with the separation. In June 2015, both parties amended and restated the BMS Collaboration Agreement (the “Amended BMS Collaboration Agreement”) to narrow the focus of the collaboration to exclusively work on anti-B-cell maturation antigen (“BCMA”) product candidates for a new three-year term. In connection with the Amended BMS Collaboration Agreement, the Company received an up-front, non-refundable, non-creditable payment of $25.0 million to fund research and development under the collaboration. On a product candidate-by-product candidate basis, up through a specified period following enrollment of the first patient in an initial Phase 1 clinical trial for such product candidate, BMS had an option to obtain an exclusive worldwide license to develop and commercialize such product. Following BMS’s license of each product candidate, the Company is entitled to elect to co-develop and co-promote each product candidate in the United States. BMS Ide-cel related agreements In February 2016, BMS exercised its option to obtain an exclusive worldwide license to develop and commercialize ide-cel, the first product candidate under the Amended BMS Collaboration Agreement, pursuant to an executed license agreement (“Ide-cel License Agreement”) and paid the Company the associated $10.0 million option fee. Pursuant to the Ide-cel License Agreement, BMS was responsible for development and related funding of ide-cel after the substantial completion of the Phase 1 clinical trial. The Company was responsible for the manufacture of vector and associated payload throughout development and, upon BMS’s request, throughout commercialization, the costs of which were reimbursable by BMS in accordance with the terms of the Amended and Restated Co-Development, Co-Promote and Profit Share Agreement, as further described below. BMS was responsible for the manufacture of drug product throughout development and commercialization. Under the Ide-cel License Agreement, the Company was eligible to receive (i) U.S. milestones of up to $85.0 million for the first indication to be addressed by ide-cel and royalties for U.S. sales of ide-cel and (ii) ex-U.S. milestones of up to $55.0 million and royalties for ex-U.S. sales of ide-cel. In March 2018, the Company elected to co-develop and co-promote ide-cel within the United States pursuant to the execution of the Amended and Restated Co-Development, Co-Promote and Profit Share Agreement (“Ide-cel CCPS”), which replaced the Ide-cel License Agreement. As a result of executing the Ide-cel CCPS, the Company will share equally in all profits and losses relating to developing, commercializing and manufacturing ide-cel within the United States and has the right to participate in the development and promotion of ide-cel in the United States. BMS is responsible for the costs incurred to manufacture vector and associated payload for use outside of the United States, plus a markup. As a result of electing to co-develop and co-promote ide-cel within the United States, the milestones and royalties payable under the Ide-cel License Agreement were adjusted. Under the Ide-cel CCPS, the Company was eligible to receive a $10.0 million milestone related to the development of ide-cel in the United States and, for the first indication to be addressed by ide-cel, ex-U.S. regulatory and commercial milestones of up to $60.0 million. Under the Ide-cel CCPS, the $10.0 million milestone related to the development of ide-cel in the United States was achieved in the second quarter of 2019 and subsequently paid by BMS. BMS bb21217 License Agreement In September 2017, BMS exercised its option to obtain an exclusive worldwide license to develop and commercialize bb21217, an investigational BCMA-targeted CAR T cell therapy, the second product candidate under the Amended BMS Collaboration Agreement, pursuant to an executed license agreement (“bb21217 License Agreement”) and paid the Company an option fee of $15.0 million. Pursuant to the bb21217 License Agreement, BMS is responsible for development and related funding of bb21217 after the substantial completion of the ongoing Phase 1 clinical trial. In 2019, the parties amended the protocol for the ongoing Phase 1 clinical trial to enroll additional patients for which the Company will be reimbursed based upon an agreed-upon amount per patient. Under the bb21217 License Agreement, the Company is eligible to receive U.S. milestones of up to $85.0 million for the first indication to be addressed by bb21217 and royalties for U.S. sales of bb21217. Additionally, the Company was eligible to receive ex-U.S. milestones of up to $55.0 million and royalties for ex-U.S. sales of bb21217. May 2020 Amendments to Ide-cel and bb21217 agreements In May 2020, the First Amendment to the Amended and Restated Co-Development, Co-Promote and Profit Share Agreement (as amended, the "Amended Ide-cel CCPS") and the Second Amended and Restated License Agreement ("Amended bb21217 License Agreement" and, collectively with the Amended Ide-cel CCPS, the “May 2020 Amendments”), which replaced the bb21217 License Agreement, was executed. Under the Amended Ide-cel CCPS, the parties will continue to share equally in all profits and losses related to developing, commercializing and manufacturing ide-cel within the United States. However, the Amended Ide-cel CCPS changed the Company’s responsibilities with respect to manufacturing activities. Under the Amended Ide-cel CCPS, BMS assumed the contract manufacturing agreements related to ide-cel adherent lentiviral vector. Over time, BMS is also assuming responsibility for manufacturing ide-cel suspension lentiviral vector outside of the United States, with the Company responsible for manufacturing ide-cel suspension lentiviral vector in the United States. In June 2023, the Company assigned its Commercial Supply Agreement with Resilience to BMS, resulting in BMS assuming responsibility for all ide-cel suspension lentiviral vector manufacturing. The May 2020 Amendments relieved BMS of its obligations to pay the Company for future ex-U.S. milestones and royalties on ex-U.S. sales for each of ide-cel and bb21217 in exchange for an up-front, non-refundable, non-creditable payment of $200.0 million, which represents the aggregate of the probability-weighted, net present value of the future ex-U.S. milestones and royalties on ex-U.S. sales for each of ide-cel and bb21217. In addition, the parties are released from future exclusivity related to BCMA-directed T cell therapies. There are no remaining milestones or royalties under the Amended Ide-cel CCPS. In March 2021, the FDA approved the marketing of Ide-cel as Abecma in the United States for the treatment of adult patients with relapsed or refractory multiple myeloma after four or more prior lines of therapy, including an immunomodulatory agent, a proteasome inhibitor, and an anti-CD38 monoclonal antibody. Under the Amended Ide-cel CCPS, BMS is primarily responsible for the commercialization of Abecma and the Company has concluded BMS is the principal for such activities for purposes of applying its ASC 808 accounting policy to the Amended Ide-cel CCPS. As previously described, under the collaboration arrangement with BMS, the Company had an option to co-develop and co-promote bb21217 within the United States. However, following completion of the CRB-402 clinical trial, in January 2022 the Company, along with BMS, evaluated its plans with respect to bb21217. Based in part on the strength of Abecma clinical data and commercial sales to date, the Company and BMS elected to discontinue development of bb21217 and, as such, the Company did not exercise its option to co-develop and co-promote bb21217 within the United States. The Company is still eligible to receive U.S. milestones and royalties for U.S. sales of bb21217, if further developed by BMS. Additionally, pursuant to the terms of the collaboration agreement, because it did not exercise its option to co-develop and co-promote bb21217, the Company received an additional fee in the amount of $10.0 million from BMS during the second quarter of 2022. Accounting Analysis Amended Ide-cel CCPS and Amended bb21217 License Agreement Prior to the Amended Ide-cel CCPS and Amended bb21217 License Agreement, the Company had constrained all variable consideration related to the remaining ex-U.S. milestones and royalties for ex-U.S. sales under the Ide-cel CCPS and bb21217 License Agreement, as these milestones and royalties for ex-U.S sales were not considered to be probable. As a result of the Amended Ide-cel CCPS and Amended bb21217 License Agreement, the uncertainty associated with the previously constrained variable consideration for future ex-U.S. milestones and royalties on ex-U.S. sales for each of ide-cel and bb21217 was resolved in exchange for an up-front, non-refundable, non-creditable payment of $200.0 million. The Amended Ide-cel CCPS and Amended bb21217 License Agreement were negotiated as a package with a single commercial objective and, as such, the Amended Ide-cel CCPS and Amended bb21217 License Agreement were combined for accounting purposes and treated as a single arrangement. At the time of execution of the Amended Ide-cel CCPS and Amended bb21217 License Agreement, there was one remaining performance obligation under each agreements: a combined performance obligation of the ide-cel license and ide-cel vector manufacturing through development; and a combined performance obligation of the bb21217 license and bb21217 vector manufacturing through development, neither of which were fully satisfied. The Company concluded each performance obligation was distinct from each other as BMS can benefit from each license and associated manufacturing services separately and the respective licenses and manufacturing services do not modify one another and are not interdependent. Accordingly, the Company continues to account for each performance obligation separately. The Company allocated the $200.0 million up-front payment received in connection with the Amended Ide-cel CCPS and Amended bb21217 License Agreement to the remaining performance obligations described above based on the general allocation principles of Topic 606. The Company considered that a portion of the $200.0 million was specifically attributable to each remaining performance obligation as the amount represents the aggregate of the probability-weighted, net present value of the future ex-U.S. milestones and royalties on ex-U.S. sales for each of ide-cel and bb21217 and that each respective portion therefore (i) relates specifically to the Company's satisfaction of each of its remaining performance obligations and (ii) is representative of the amount of consideration the Company expects to be entitled to in exchange for satisfying the respective performance obligations. As such, the Company concluded that the portion of the $200.0 million up-front payment specifically attributable to each of ide-cel and bb21217 should be allocated to each respective performance obligation pursuant to the variable consideration allocation exception. The Amended Ide-cel CCPS and Amended bb21217 License Agreement represent a contract modification to an existing contract under Topic 606 resulting in a reduction in scope of the Company's responsibilities under each performance obligation described above. This resulted in a change in the overall transaction price under the arrangement. The Amended Ide-cel CCPS and Amended bb21217 License Agreement did not include any additional promised goods and services. The remaining goods and services to be provided to fully satisfy each performance obligation described above are not distinct from those previously provided with respect to each performance obligation. Therefore, for each performance obligation, the remaining goods and services are part of a single performance obligation that is partially satisfied at the date of the contract modification. Accordingly, the effect that the contract modification had on the transaction price and the measure of progress toward complete satisfaction of each respective performance obligation has been recognized on a cumulative catch-up basis. The accounting for any previously satisfied performance obligations as of the contract modification date are not affected by the modification. Ide-cel research and development services The Company allocated $40.9 million of the ide-cel transaction price to the ide-cel research and development services. The research and development performance obligation was satisfied prior to the May 2020 Amendments and, as a result, the accounting for this previously satisfied performance obligation was not affected by the modification. The Company recognized no revenue related to ide-cel research and development services for the years ended December 31, 2023, 2022, and 2021. Ide-cel license and manufacturing services The Company allocated $347.0 million of the ide-cel transaction price to the combined unit of accounting which consists of the license and manufacture of vectors and associated payload for incorporation into ide-cel through development. As of December 31, 2023, there was no unsatisfied transaction price and the remaining deferred revenue as of December 31, 2020 related to the performance obligation was recognized when the performance obligation was satisfied during the first quarter of 2021, upon Abecma commercial approval. The following table summarizes the net collaboration revenue recognized or expense incurred for the joint ide-cel development efforts in the U.S. under ASC 808, including revenue or expense related to the combined performance obligation for license and manufacturing of ide-cel in the U.S. for the years ended December 31, 2023, 2022 and 2021 (in thousands): For the years ended December 31, 2023 2022 2021 ASC 808 ide-cel license and manufacturing revenue - U.S. (1)(2) $ — $ — $ 533 ASC 808 ide-cel license and manufacturing expense - U.S. (1) $ — $ — $ — (1) As noted above, the calculation of collaborative arrangement activity to be recognized for joint ide-cel efforts in the United States is performed on a quarterly basis. The calculation is independent of previous activity, which may result in fluctuations between revenue and expense recognition period over period, depending on the varying extent of effort performed by each party during the period. Abecma Subsequent to the satisfaction of the Company’s combined performance obligation license and manufacturing of ide-cel in the U.S. under the collaboration agreement with BMS, the Company shares equally in the profit and loss related to the development and commercialization of ide-cel in the United States (marketed as Abecma ). The Company has no remaining financial rights with respect to the development or commercialization of ide-cel outside of the United States. The Company accounts for its collaborative arrangement efforts with BMS in the United States within the scope of ASC 808 given that both parties are active participants in the activities and both parties are exposed to significant risks and rewards dependent on the commercial success of the activities. The calculation of collaborative activity to be recognized for joint Abecma efforts in the United States is performed on a quarterly basis and is independent of previous quarterly activity. This may result in fluctuations between revenue and expense recognition period over period, depending on the varying extent of effort performed by each party during the period. The Company recognizes revenue related to the combined unit of accounting for the ex-U.S. license and lentiviral vector manufacturing services under Topic 606. Ide-cel U.S. Share of Collaboration Profit or Loss The U.S. commercial and development activities under the Amended Ide-Cel CCPS are within the scope of ASC 808. On a quarterly basis, the Company determines its share of collaboration profit or loss for commercial activities (i.e., commercial sales of Abecma by BMS). The Company’s share of any collaboration profit for commercial activities is recognized as collaborative arrangement revenue and its share of any collaboration loss for commercial activity is recognized as an operating expense and classified as share of collaboration loss on the Company's consolidated and combined statements of operations and comprehensive loss. The Company is also responsible for equally sharing in the ongoing ide-cel research and development activities being conducted by BMS in the United States as BMS continues conducting ongoing clinical studies to support the use of Abecma in earlier lines of therapy. The net amount owed to BMS for research and development activities determined on a quarterly basis is classified as research and development expense on the statements of operations and comprehensive loss. If BMS is obligated to reimburse the Company because the Company’s research and development costs exceeds BMS’ research and development costs in a particular quarterly period, the net amount is recorded as collaborative arrangement revenue. The following tables summarize the components utilized in the Company’s quarterly calculation of collaborative arrangement revenue or share of collaboration loss under the BMS collaboration arrangement for the years ended December 31, 2023, 2022 and 2021 (in thousands). The amounts reported for these periods represent the Company’s share of BMS’ Abecma product revenue, cost of goods sold, and selling costs, along with reimbursement by BMS of commercial costs incurred by the Company, and exclude expenses related to ongoing development, which are separately reflected in the consolidated and combined statements of operations and comprehensive loss as described below. Three months ended Year ended Abecma U.S. Collaboration Profit/Loss Share March 31, 2023 June 30, 2023 September 30, 2023 December 31, 2023 December 31, 2023 2seventy's share of profits (losses), net of 2seventy's share of BMS costs for commercial activities $ 21,581 $ 23,272 $ (582) $ 1,366 $ 45,637 Reimbursement from BMS for 2seventy costs of commercial manufacturing and commercial activities 1,380 1,271 1,118 604 4,373 Collaborative arrangement revenue (1) $ 22,961 $ 24,543 $ 536 $ 1,970 $ 50,010 Share of collaboration loss (1) $ — $ — $ — $ — $ — Three months ended Year ended Abecma U.S. Collaboration Profit/Loss Share March 31, 2022 June 30, 2022 September 30, 2022 December 31, 2022 December 31, 2022 2seventy's share of profits (losses), net of 2seventy's share of BMS costs for commercial activities $ (6,709) $ (5,931) $ 2,849 $ 7,286 $ (2,505) Reimbursement from BMS for 2seventy costs of commercial manufacturing and commercial activities 1,357 1,641 1,215 1,431 5,644 Collaborative arrangement revenue (1) $ — $ — $ 4,064 $ 8,717 $ 12,781 Share of collaboration loss (1) $ (5,352) $ (4,290) $ — $ — $ (9,642) Three months ended Year ended Abecma U.S. Collaboration Profit/Loss Share March 31, 2021 (2) June 30, 2021 September 30, 2021 December 31, 2021 December 31, 2021 2seventy's share of profits (losses), net of 2seventy's share of BMS costs for commercial activities $ — $ (11,766) $ 9,762 $ 7,901 $ 5,897 Reimbursement from BMS for 2seventy costs of commercial manufacturing and commercial activities — 1,695 845 917 3,457 Collaborative arrangement revenue (1) $ — $ — $ 10,607 $ 8,818 $ 19,425 Share of collaboration loss (1) $ — $ (10,071) $ — $ — $ (10,071) (1) As noted above, the calculation is performed on a quarterly basis and consists of 2seventy's share of profits, net of 2seventy's share of BMS costs for commercial activities, offset by reimbursement from BMS for 2seventy commercial activities. The calculation is independent of previous activity, which may result in fluctuations between revenue and expense recognition period over period. (2) In March 2021, Abecma (idecabtagene vicleucel; ide-cel) was approved by the FDA in the United States for the treatment of adults with multiple myeloma who have received at least four prior lines of therapy, including an immunomodulatory agent, a proteasome inhibitor, and an anti-CD38 monoclonal antibody. The first sale of Abecma occurred in the second quarter of 2021. Collaborative arrangement revenue net of share of collaboration loss was $50.0 million, $3.1 million and $9.4 million for the years ended December 31, 2023, 2022, and 2021, respectively. The following table summarizes the amounts associated with the research activities under the collaboration included in research and development expense or recognized as collaborative arrangement revenue for each quarter during the years ended December 31, 2023, 2022, and 2021 (in thousands): Three months ended Year ended Abecma U.S. Collaboration Net R&D Expenses March 31, 2023 June 30, 2023 September 30. 2023 December 31, 2023 December 31, 2023 2seventy's obligation for its share of BMS research and development expenses $ (9,461) $ (7,195) $ (6,980) $ (7,237) $ (30,873) Reimbursement from BMS for 2seventy research and development expenses 4,590 1,543 860 151 7,144 Net R&D expense (1) $ (4,871) $ (5,652) $ (6,120) $ (7,086) $ (23,729) Three months ended Year ended Abecma U.S. Collaboration Net R&D Expenses March 31, 2022 June 30, 2022 September 30. 2022 December 31, 2022 December 31, 2022 2seventy's obligation for its share of BMS research and development expenses $ (8,118) $ (7,418) $ (10,672) $ (8,197) $ (34,405) Reimbursement from BMS for 2seventy research and development expenses 1,225 1,955 1,420 1,960 6,560 Net R&D expense (1) $ (6,893) $ (5,463) $ (9,252) $ (6,237) $ (27,845) Three months ended Year ended Abecma U.S. Collaboration Net R&D Expenses March 31, 2021 June 30, 2021 September 30. 2021 December 31, 2021 December 31, 2021 2seventy's obligation for its share of BMS research and development expenses $ (22,110) $ (11,559) $ (9,398) $ (8,780) $ (51,847) Reimbursement from BMS for 2seventy research and development expenses 4,752 2,366 3,738 945 11,801 Net R&D expense (1) $ (17,358) $ (9,193) $ (5,660) $ (7,835) $ (40,046) (1) As noted above, the calculation is performed on a quarterly basis and consists of 2seventy's obligation for its share of BMS research and development expenses, offset by reimbursement from BMS for 2seventy research and development expenses. Ide-cel ex-U.S. Service Revenue The Company accounts for any ex-U.S. activities under the Amended Ide-cel CCPS pursuant to ASC 606. The following table summarizes the revenue recognized related to ide-cel ex-U.S. activities for the years ended December 31, 2023, 2022, and 2021 (in thousands). These amounts are reflected in service revenue in the consolidated and combined statements of operations and comprehensive loss: For the years ended December 31, 2023 2022 2021 ASC 606 ide-cel license and manufacturing revenue – ex-U.S. (included as a component of service revenue) (1) $ 14,751 $ 13,226 $ 16,895 (1) These amounts include reimbursements from BMS to the Company for the Company’s ex-U.S. quality and other manufacturing costs associated with the manufacture of Abecma inventory. bb21217 research and development services The Company allocated $5.4 million of the bb21217 transaction price to the research and development services. The research and development performance obligation was satisfied prior to the May 2020 amendments, and as a result, the accounting for this previously satisfied performance obligation was not affected by the modification. The Company recognized no revenue related to bb21217 research and developments services for the years ended December 31, 2023, 2022, and 2021. bb21217 license and manufacturing services The Company allocated $35.5 million of the bb21217 transaction price to its combined performance obligation representing the bb21217 license and vector manufacturing services. Of this amount, $1.8 million was never received by the Company as it represented the estimated reimbursement from BMS for the manufacture of vectors and associated payload through development. As noted above, following completion of the CRB-402 clinical trial for bb21217, in January 2022 the Company, along with BMS, evaluated its plans with respect to bb21217. Based in part on the strength of Abecma clinical data and commercial sales to date, the Company and BMS elected to discontinue development of bb21217 and, as such, the Company did not exercise its option to co-develop and co-promote bb21217 within the United States. Additionally, pursuant to the terms of the collaboration agreement, because it did not exercise its option to co-develop and co-promote bb21217, the Company received an additional fee in the amount of $10.0 million from BMS during the second quarter of 2022. Pursuant to the variable consideration allocation exception, the $10.0 million of consideration received was allocated to the combined performance obligation for the bb21217 license and vector manufacturing services through development, described above. In December 2022, BMS formally notified the Company that its license and vector manufacturing services for bb21217 will no longer be required, thus releasing it from the combined performance obligation for the bb21217 license and vector manufacturing services through development. As a result, the Company recognized the remaining deferred revenue of $35.8 million as a component of service revenue during the year ended December 31, 2022. Contract assets and liabilities – ide-cel and bb21217 The Company receives payments from its collaborative partners based on billing schedules established in each contract. Up-front payments and fees are recorded as deferred revenue upon receipt or when due until such time as the Company satisfies its performance obligations under these arrangements. A contract asset is a conditional right to consideration in exchange for goods or services that the Company has transferred to a customer. Amounts are recorded as accounts receivable when the Company’s right to consideration is unconditional. The following table presents changes in the balances of the Company’s BMS receivables and contract liabilities during the twelve months ended December 31, 2023 (in thousands): Balance at December 31, Additions Deductions Balance at December 31, Receivables $ 4,537 $ 46,212 $ (50,749) $ — Contract liabilities: Deferred revenue $ — $ — $ — $ — The decrease in the receivables balance for the twelve months ended December 31, 2023 is driven by amounts owed to the Company from BMS in the period under the settlement terms of the collaboration agreement, offset by amounts collected from BMS in the period. Regeneron Regeneron Collaboration Agreement In August 2018, bluebird bio entered into a Collaboration Agreement (the “Regeneron Collaboration Agreement”) with Regeneron pursuant to which the parties will apply their respective technology platforms to the discovery, development, and commercialization of novel immune cell therapies for cancer. As noted above, the agreement was attributed to the Company in connection with the separation. Under the terms of the agreement, the parties will leverage Regeneron’s proprietary platform technologies for the discovery and characterization of fully human antibodies, as well as T cell receptors directed against tumor-specific proteins and peptides and the Company will contribute its field-leading expertise in gene therapy. In accordance with the Regeneron Collaboration Agreement, the parties jointly selected six initial targets and intend to equally share the costs of research up to the point of submitting an IND application for a potential gene therapy product directed to a particular target. Additional targets may be selected to add to or replace any of the initial targets during the five-year research collaboration term as agreed to by the parties. Regeneron will accrue a certain number of option rights exercisable against targets as the parties reach certain milestones under the terms of the agreement. Upon the acceptance of an IND for the first product candidate directed to a target, Regeneron will have the right to exercise an option for co-development/co-commercialization of product candidates directed to such target on a worldwide or applicable opt-in territory basis, with certain exceptions. Where Regeneron chooses to opt-in, the parties will share equally in the costs of development and commercialization, and will share equally in any profits or losses therefrom in applicable opt-in territories. Outside of the applicable opt-in territories, the target becomes a licensed target and Regeneron would be eligible to receive, with respect to any resulting product, milestone payments of up to $130.0 million per product and royalties on net sales outside of the applicable opt-in territories at a rate ranging from the mid-single digits to low-double digits. A target would also become a licensed target in the event Regeneron does not have an option to such target, or Regeneron does not exercise its option with respect to such target. Either party may terminate a given research program directed to a particular target for convenience, and the other party may elect to continue such research program at its expense, receiving applicable cross-licenses. The terminating party will receive licensed product royalties and milestone payments on the potential applicable gene therapy products. Where the Company terminates a given research program for convenience, and Regeneron elects to continue such research program, the parties will enter into a transitional services agreement. Under certain conditions, following its opt-in, Regeneron may terminate a given collaboration program and the Company may elect to continue the development and commercialization of the applicable potential gene therapy products as licensed products. First Amendment to the Regeneron Collaboration Agreement In January 2023, 2seventy bio and Regeneron announced an amendment to the Regeneron Collaboration Agreement (the “Amendment”), to amend and extend their current agreement, applying their respective technology platforms to the discovery, development and commercialization of novel immune cell therapies for cancer. Under the Amendment, the parties have identified four research targets to advance the next stage of research therapies. The parties will continue sharing costs for these activities in a manner largely consistent with the existing agreement, with Regeneron now covering 75% of eligible late-stage research costs to study combinations and 100% of the costs for the arms of clinical studies that include Regeneron agents through regulatory approval of two of the four targets. For other programs, cost-sharing will follow the existing 50/50 cost sharing agreement. Additionally, Regeneron will make one-time milestone payments for each of the first Clinical Candidate directed to MUC16 and the first Clinical Candidate directed to a selected early stage research target to achieve the applicable milestones. Clinical Candidate milestone events and payments include: • $2.0 million payment from Regeneron for Development Candidate Nomination; • $3.0 million payment from Regeneron for IND Acceptance; and • $5.0 million payment from Regeneron for the Earlier of (i) last patient dosed with a Monotherapy Regimen and (ii) dosing of the 10th patient in a Clinical Tria

Royalty and other revenue

Royalty and other revenue	12 Months Ended
Royalty and other revenue	Dec. 31, 2023
License And Royalty Revenue [Abstract]
Royalty and other revenue	Royalty and other revenue bluebird bio has out-licensed intellectual property to various third parties. Under the terms of these agreements, some of which were assumed by the Company in connection with the separation, bluebird bio and the Company may be entitled to royalties and milestone payments. The Company recognized $4.6 million, $3.6 million, and $6.2 million of royalty and other revenue in the years ended December 31, 2023, 2022, and 2021, respectively. Novartis Pharma AG In April 2017, bluebird bio entered into a worldwide license agreement with Novartis. Under the terms of the agreement, Novartis non-exclusively licensed certain patent rights related to lentiviral vector technology to develop and commercialize CAR T cell therapies for oncology, including Kymriah (formerly known as CTL19), Novartis’s anti-CD19 CAR T therapy. The agreement was assumed by the Company in connection with the separation. Beginning in the fourth quarter of 2017, bluebird bio began receiving royalties from sales of tisagenlecleucel under the agreement. This license agreement was terminated effective March 2021, at which point in time Novartis was no longer required to make royalty or other payments on net sales of tisagenlecleucel or any future products. Royalty revenue recognized from sales of tisagenlecleucel is included within royalty and other revenue in the condensed consolidated and combined statement of operations and comprehensive loss. Juno Therapeutics In May 2020, bluebird bio entered into a non-exclusive license agreement with Juno Therapeutics, Inc. (“Juno”), a wholly-owned subsidiary of BMS, related to lentiviral vector technology to develop and commercialize CD-19-directed CAR T cell therapies. The agreement was assumed by the Company in connection with the separation. Upon regulatory approval of Breyanzi (lisocabtagene maraleucel) during the first quarter of 2021, bluebird bio received a $2.5 million milestone payment from Juno, which is included within royalty and other revenue in the Company’s consolidated and combined financial statements. Royalty revenue recognized from sales of Breyanzi (lisocabtagene maraleucel) through August 2023, the end of the royalty term, is also included within royalty and other revenue in the consolidated and combined statement of operations and comprehensive loss.

Intangible assets

Intangible assets	12 Months Ended
Intangible assets	Dec. 31, 2023
Goodwill and Intangible Assets Disclosure [Abstract]
Intangible assets	Intangible assets Intangible assets, net of accumulated amortization, are summarized as follows (in thousands): As of December 31, As of December 31, 2023 2022 Cost Accumulated amortization Net Cost Accumulated amortization Net Developed technology $ 30,100 $ (30,100) $ — $ 30,100 $ (30,100) $ — In-licensed rights 8,500 (1,906) 6,594 8,500 (1,198) 7,302 Total $ 38,600 $ (32,006) $ 6,594 $ 38,600 $ (31,298) $ 7,302 Amortization expense for intangible assets was $0.7 million, $2.6 million, and $4.3 million for the years ended December 31, 2023, 2022 and 2021, respectively. Developed technology The Company's developed technology was obtained through the acquisition in 2014 of Pregenen, a privately-held biotechnology company includes gene editing and cell signaling technology with a broad range of potential therapeutic applications. The gene editing platform intangible asset was amortized on a straight-line basis over its expected useful life until it was fully amortized in the second quarter of 2022. In-licensed rights In-licensed rights consist of capitalized milestone payments made to third parties upon receiving regulatory approval of Abecma in the U.S. The in-licensed rights are being amortized on a straight-line basis over the remaining life of the related patents of approximately twelve years, as the life of the related patents reflects the expected time period that the Company will benefit from the in-licensed rights. The following table summarizes the estimated future amortization for intangible assets for the next five years and thereafter (in thousands): As of December 31, 2023 2024 $ 708 2025 708 2026 708 2027 708 2028 708 2029 and thereafter 3,054 Total $ 6,594

Stock-based compensation

Stock-based compensation	12 Months Ended
Stock-based compensation	Dec. 31, 2023
Share-Based Payment Arrangement [Abstract]
Stock-based compensation	Stock-based compensation The Company’s employees have historically participated in bluebird bio’s various stock-based compensation plans. In connection with 2seventy’s separation from bluebird bio on November 4, 2021, under the provisions of the existing plans, the outstanding bluebird bio equity awards were adjusted in accordance with the terms of the Employee Matters Agreement (equitable adjustment) to preserve the intrinsic value of the awards immediately before and after distribution. Upon the distribution, employees holding stock options, restricted stock units (“RSUs”) and performance restricted stock units (“PRSUs”) denominated in pre-distribution bluebird bio stock received a number of otherwise-similar awards either in post-distribution 2seventy stock or in a combination of post-distribution bluebird stock and 2seventy stock based on conversion ratios outlined for each group of employees in the Employee Matters Agreement that the Company entered into in connection with the distribution. The equity awards that were granted prior to 2021 were converted under the shareholder method, wherein employees holding outstanding equity awards received equity awards in both bluebird and 2seventy. The conversion ratio for the shareholder method took into consideration a distribution ratio of one share of 2seventy common stock for every three shares of bluebird bio common stock. For equity awards granted in 2021, the number of awards that were outstanding at the time of the spin-off were proportionately adjusted to maintain the aggregate intrinsic value of the awards at the date of the spin-off. The conversion ratio was determined based on the volume weighted-average trading price for bluebird common stock for the five trading days before and the volume weighted-average trading price for 2seventy common stock for the five trading days after the spin. These modified awards otherwise retained substantially the same terms and conditions, including term and vesting provisions. Due to the modification of the equity awards as a result of the distribution, the Company compared the fair value of the outstanding equity awards immediately before and after the distribution. The modification resulted in an incremental fair value of $1.5 million, of which $1.3 million was immediately recognized as of the distribution. The Company will incur future compensation cost related to bluebird bio equity awards held by its employees but will not incur future compensation cost related to bluebird employees holding equity awards in 2seventy. In October 2021, the Company’s board of directors adopted its 2021 Stock Option and Incentive Plan (“2021 Plan”), which was subsequently approved by bluebird bio, then the Company’s sole stockholders. The 2021 Plan became effective on October 17, 2021, the day immediately prior to the effectiveness of the Company’s Registration Statement on Form 10. The 2021 Plan allows for the granting of incentive stock options, non-qualified stock options, RSUs, PRSUs, and restricted stock awards to 2seventy bio’s employees, members of the board of directors, and consultants of 2seventy bio, including those of 2seventy bio who became employees of the Company in connection with the separation. All awards granted under the 2021 Plan consist of shares of 2seventy bio’s common stock. Stock-based compensation expense For periods prior to the separation, stock-based compensation expense was allocated to the Company using a combination of specific identification and time spent on projects at various levels of the organization, which management believes are consistent and reasonable. The Company recognized stock-based compensation expense totaling $32.2 million, $41.0 million, and $54.6 million during the years ended December 31, 2023, 2022, and 2021, respectively. Stock-based compensation expense recognized by award type is as follows (in thousands): Year Ended December 31, 2023 2022 2021 Stock options $ 11,217 $ 17,784 $ 26,185 Restricted stock units 20,607 22,983 21,414 Employee stock purchase plan and other 335 273 7,030 $ 32,159 $ 41,040 $ 54,629 Stock-based compensation expense by classification included within the consolidated statements of operations and comprehensive loss was as follows (in thousands): Year Ended December 31, 2023 2022 2021 Research and development $ 12,508 $ 18,333 $ 29,746 Selling, general and administrative 19,651 22,707 24,883 $ 32,159 $ 41,040 $ 54,629 As of December 31, 2023, the Company had $14.6 million, $19.0 million, and $0.0 million of unrecognized compensation expense related to unvested stock options, RSUs, and PRSUs, respectively, that is expected to be recognized over a weighted-average period of 2.4 years, 2.1 years, and 0.0 years, respectively. Stock options The fair value of each option issued to employees was estimated at the date of grant using the Black-Scholes option pricing model with the following weighted average assumptions: Year ended December 31, Year ended December 31, 2023 2022 Expected volatility 80.9 % 78.9 % Expected term (in years) 5.9 6.1 Risk-free interest rate 4.2 % 1.8 % Expected dividend yield 0.0 % 0.0 % (a) In connection with 2seventy’s separation from bluebird bio on November 4, 2021, all option awards granted prior to 2022 were converted as part of the separation. The following table summarizes the stock option activity under the Company’s equity award plans for 2seventy employees: Shares Weighted-average exercise price per share Weighted-average contractual life (in years) Aggregate intrinsic value (a) (in thousands) Outstanding at December 31, 2022 2,285 $ 53.72 Granted 2,024 $ 9.52 Exercised — $ — Cancelled or forfeited (830) $ 75.11 Outstanding at December 31, 2023 3,479 $ 22.88 8.3 $ 232 Exercisable at December 31, 2023 1,113 $ 47.51 6.8 $ — Vested and expected to vest at December 31, 2023 3,479 $ 22.88 8.3 $ 232 (a) The aggregate intrinsic value is calculated as the difference between the exercise price of the underlying options and the fair value of the common stock for the options that were in the money at December 31, 2023. Restricted stock units The following table summarizes the restricted stock unit activity under the Company’s equity award plans for 2seventy employees: Shares Weighted-average grant date Unvested balance at December 31, 2022 1,270 $ 28.31 Granted 1,902 12.30 Vested (570) 34.82 Forfeited (371) 19.06 Unvested balance at December 31, 2023 2,231 $ 14.54 The intrinsic value of RSUs vested during the year ended December 31, 2023 was $4.5 million. On February 1, 2021, the Company’s CEO was granted 27,000 PRSUs by bluebird. Each PRSU related to one share of common stock of bluebird. The number of PRSUs earned and vested would be determined based on bluebird’s total shareholder return (“TSR”) compared against the median TSR of a peer group over the performance period January 1, 2021 through December 31, 2023. Upon spin-off, the PRSUs were converted to 17,387 2seventy PRSUs. The TSR was kept whole by treating the value of bluebird at spin as a dividend that is reinvested into 2seventy stock. For the years ended December 31, 2023, 2022, and 2021, the Company recognized $0.6 million, $0.6 million, and $0.4 million of expense, respectively, related to the PRSUs. As of December 31, 2023, all expense related to the PRSUs has been recognized. Based on the comparison of bluebird’s TSR from January 1, 2021 to the spin-off date and 2seventy’s TSR from the spin-off date to December 31, 2023 to the median TSR of the peer group, the number of PRSUs earned and vested during the performance period was determined to be zero. Unrestricted stock awards During the fourth quarter of 2021, the Company granted 0.2 million unrestricted stock awards to employees as part of its 2021 retention program which was designed to incentivize and retain employees through the spin-off. Under the retention program, employees were entitled to a one-time bonus payment, consisting of both a cash payment and unrestricted stock awards, with the condition that the employee remained employed through the end of 2021. Expense recognized on the unrestricted stock awards during the year ended December 31, 2021 totaled $6.5 million. The Company recognized immaterial expense associated with unrestricted stock awards during the year ended December 31, 2022 and no expense associated with unrestricted stock awards during the year ended December 31, 2023. Employee Stock Purchase Plan In October 2021, the Company’s board of directors adopted its 2021 Employee Stock Purchase Plan (“2021 ESPP”), which was subsequently approved by bluebird bio, then its sole stockholder. The 2021 ESPP became effective on October 17, 2021, the day immediately prior to the effectiveness of the Company’s Registration Statement on Form 10. The 2021 ESPP authorizes the initial issuance of a specified number of shares of the Company’s common stock to participating employees. During each of the years ended December 31, 2023 and December 31, 2022, 0.1 million shares and less than 0.1 million shares of common stock were issued under the 2021 ESPP, respectively.

Related-party transactions

Related-party transactions	12 Months Ended
Related-party transactions	Dec. 31, 2023
Related Party Transactions [Abstract]
Related-party transactions	Related-party transactions Relationship with Bluebird Bio In connection with the separation, the Company entered into a separation agreement (the “Separation Agreement”) with bluebird bio, dated as of November 3, 2021, that, among other things, set forth bluebird bio’s agreements with 2seventy bio regarding the principal actions to be taken in connection with the separation, including the distribution. The effective time of the distribution was 12:01 a.m. on November 4, 2021. The Separation Agreement identifies assets transferred to, liabilities assumed by and contracts assigned to 2seventy bio as part of the separation, and it provides for when and how these transfers, assumptions and assignments occur. Each of 2seventy bio and bluebird bio agreed to releases, with respect to pre-separation claims, and cross indemnities with respect to post-separation claims, that are principally designed to place financial responsibility for the obligations and liabilities allocated to 2seventy bio under the Separation Agreement with 2seventy bio and financial responsibility for the obligations and liabilities allocated to bluebird bio under the Separation Agreement. Following the completion of the separation and distribution, the Company and bluebird bio have operated separately, each as an independent public company and bluebird bio no longer owns any shares of the Company’s common stock. Therefore, starting in 2023, transactions under those agreements are no longer accounted for as related party transactions. The Company recorded other income of $2.8 million for the year ended December 31, 2022 and an expense of $0.2 million for the two months since separation through December 31, 2021, related to the Separation Agreement. The Company and bluebird bio also entered into a tax matters agreement, dated as of November 3, 2021, governing bluebird bio’s and 2seventy bio's respective rights, responsibilities and obligations with respect to taxes (including taxes arising in the ordinary course of business and taxes, if any, incurred as a result of any failure of the distribution and certain related transactions to qualify as tax-free for U.S. federal income tax purposes, tax attributes, the preparation and filing of tax returns, the control of audits and other tax proceedings, and assistance and cooperation in respect of tax matters). In connection with the separation, the Company also entered into an employee matters agreement with bluebird bio, dated as of November 3, 2021. The employee matters agreement allocates assets, liabilities and responsibilities relating to the employment, compensation and employee benefits of bluebird bio and 2seventy bio employees, and other related matters, in connection with the separation, including the treatment of outstanding bluebird bio incentive equity awards and certain retirement and welfare benefit obligations. The Company recorded a reduction to operating expense of $0.2 million for the year ended December 31, 2022 and $1.8 million for the two months since separation through to December 31, 2021 for costs stipulated by the employee matters agreement. The Company and bluebird bio also entered into an intellectual property license agreement on November 3, 2021, pursuant to which each party granted a license to certain intellectual property and technology to the other. bluebird bio granted 2seventy bio a perpetual, worldwide, non-exclusive, royalty-free, fully paid-up license (or, as the case may be, sublicense) to certain intellectual property to allow 2seventy bio to use such intellectual property in connection with 2seventy bio's ongoing and future research and development activities and product candidates. 2seventy bio granted bluebird bio a perpetual, worldwide, non-exclusive, royalty-free, fully paid-up license (or, as the case may be, sublicense) to certain intellectual property for use in bluebird bio’s existing products and product candidates. Such licenses between the parties generally allow current or future uses of the intellectual property in connection with each party's respective fields. As part of this agreement, the Company directly contacts and coordinates with bluebird bio’s third party licensors to make direct payments to the third parties. The Company and bluebird bio entered into two transition services agreements on November 3, 2021, pursuant to which bluebird bio will provide 2seventy bio with corporate and shared services and resources related to corporate functions, and to which 2seventy bio will provide certain services to bluebird bio, each for an initial term of two years, unless earlier terminated or extended according to the terms of the transition services agreement. For the year ended December 31, 2022, the Company recorded $10.0 million in other income, net, reflecting services provided to bluebird bio and $1.0 million of operating expenses for services received from bluebird bio. For the two months since separation through December 31, 2021, the Company recorded $1.4 million in other income, net and $0.7 million of operating expenses for services provided and received from bluebird bio, respectively. Additionally, under the transition services agreements, 2seventy bio was subleasing 30% of its headquarters at 60 Binney Street in Cambridge, Massachusetts to bluebird bio through the first quarter of 2022. Beginning in the second quarter of 2022, this percentage decreased to 23% for the remainder of the year. The Company recorded $4.9 million for the year ended December 31, 2022 and $0.8 million for the year ended December 31, 2021 in other income, net related to sublease income from bluebird under this arrangement. As of December 31, 2022, amounts due to bluebird bio under the above agreements were $0.1 million and are included in accrued expenses. As of December 31, 2022, amounts due from bluebird bio under the above agreements were $1.9 million and were included in receivables and other current assets. On February 23, 2023, the Company entered into a Partial Assignment and Assumption Agreement (the “Assignment and Assumption Agreement”) with Institut Pasteur (“Institut Pasteur”) and bluebird bio. Pursuant to the Assignment and Assumption Agreement, bluebird bio assigned to the Company bluebird bio’s rights, obligations and interests under a license agreement with Institut Pasteur that were previously licensed to the Company by bluebird bio under the License Agreement. The Company will pay Institut Pasteur an annual maintenance payment, a percentage of income received in the event of sublicensing arrangements and, upon commercialization of certain products, a percentage of net sales as a royalty, which varies depending on the indication of the product. Corporate allocations Prior to the separation, the Company did not operate as a separate, stand-alone entity, but rather was managed and operated in the normal course of business under bluebird bio. Accordingly, certain shared costs have been allocated to the Company and reflected as expenses in the Company's stand-alone consolidated and combined financial statements for periods prior to the separation. The expenses reflected in the consolidated and combined financial statements may not be indicative of expenses that will be incurred by the Company in the future. For the year ended December 31, 2021, the consolidated and combined financial statements reflect allocations of certain expenses from bluebird bio, including, but not limited to, general corporate expenses, such as senior management, legal, human resources, accounting, other financial services (such as treasury, audit and purchasing), tax, information technology, and corporate employee benefits, incentives and stock-based compensation included within selling, general and administrative expense. These expenses have been allocated to the Company based on direct usage or benefit where specifically identifiable, with the remainder allocated based on employee time spent on projects, square footage or other measures that management believes are consistent and reasonable. Allocations for management costs and corporate support services provided to the Company totaled $55.3 million, for the year ended December 31, 2021. The financial information in these consolidated and combined financial statements for periods prior to the separation does not necessarily include all the expenses that would have been incurred by the Company had it been a separate, stand-alone entity. Actual costs that may have been incurred if the Company had been a stand-alone company would depend on a number of factors, including the chosen organization structure and functions outsourced or performed by employees. See Note 2, Summary of significant accounting policies and basis of presentation , for additional information on the preparation and basis of presentation of these consolidated and combined financial statements, including the treatment of certain research and development costs not directly attributable to individual programs. Usage of the Company's assets by bluebird bio and of bluebird bio's assets by the Company prior to the separation Certain assets have been reflected in these consolidated and combined financial statements as the underlying assets were attributed to the Company; however, bluebird bio has historically utilized a portion of the underlying asset as part of its operations. Accordingly, the expense related to the underlying asset has been reflected in the combined financial statements. The Company has also recorded an imputed charge to bluebird bio to reflect the cost of bluebird bio's proportional usage. In addition, the Company has recorded as an expense an imputed charge to reflect the cost of the Company's proportional usage of certain underlying assets not reflected in the consolidated and combined financial statements but for which the Company has historically utilized a portion of the underlying asset as part of its operations. The income and expense recognized by the Company for periods prior to the separation resulting from these imputed charges was recorded as other income, net in the combined financial statements and was as follows: Year ended December 31, 2021 Imputed charge to bluebird bio for leases $ 14,833 Imputed charge from bluebird bio for leases (908) Imputed charge to bluebird bio for property, plant and equipment 1,891 Imputed charge from bluebird bio for property, plant and equipment (1,130) Imputed charge to bluebird bio for intangible assets 82 Other (1) $ 14,767 Other components of other income, net, that are not shown in the table above include immaterial gains and losses on disposals of fixed assets. Stock-based compensation As discussed in Note 13, Stock-based compensation , prior to the separation, 2seventy bio’s employees participated in bluebird bio's stock-based compensation plans, the costs of which have been allocated to 2seventy bio and recorded in research and development and selling, general and administrative expenses in the consolidated and combined statements of operations and comprehensive loss. Retirement plans As discussed in Note 15, 401(k) Savings plan , prior to the separation, 2seventy bio’s employees participated in bluebird bio's 401(k) Savings plan, the costs of which have been allocated to 2seventy bio and recorded in research and development and selling, general and administrative expenses in the consolidated and combined statements of operations and comprehensive loss. Transaction costs Prior to the separation, bluebird bio had incurred costs related to the separation of the Company. To the extent separation costs are incurred that will directly benefit the Company as a stand-alone company, such costs were allocated to the Company. Centralized cash management Prior to the separation, no separate cash accounts for 2seventy bio were maintained and, therefore, bluebird bio was presumed to have funded 2seventy bio’s operating, investing and financing activities as necessary. As cash was disbursed and received by bluebird bio, for purposes of the consolidated and combined financial statements, funding of 2seventy bio’s expenditures was reflected in the consolidated and combined financial statements as a component of net parent investment.

401(k) Savings plan

401(k) Savings plan	12 Months Ended
401(k) Savings plan	Dec. 31, 2023
Retirement Benefits [Abstract]
401(k) Savings plan	401(k) Savings plan Prior to the spin-off, all 2seventy employees were covered by bluebird bio’s defined-contribution savings plan under Section 401(k) of the Internal Revenue Code (“bluebird 401(k) Plan”), which was established in 1997. Upon spin-off, 2seventy bio established a defined-contribution savings plan under Section 401(k) of the Internal Revenue Code (“2seventy 401(k) Plan”), which covers all employees who meet defined minimum age and service requirements, including those who became employees of the Company, and allows participants to defer a portion of their annual compensation on a pretax basis. Expense related to the bluebird and 2seventy 401(k) Plans for 2seventy employees totaled $3.1 million, $3.2 million, and $2.4 million for the years ended December 31, 2023, 2022, and 2021, respectively.

Income taxes

Income taxes	12 Months Ended
Income taxes	Dec. 31, 2023
Income Tax Disclosure [Abstract]
Income taxes	Income taxes The Company did not operate as a separate, stand-alone entity for periods prior to the separation. The Company’s statement of operations for periods prior to separation have been prepared on a carve out basis. The Company’s income tax provision for the years ended December 31, 2023 and December 31, 2022 and for the period from November 4, 2021 through December 31, 2021 has been prepared on a stand-alone basis. The 2021 component of loss before income taxes and rate reconciliation is presented for the full year period of activity, including for the portion prior to the separation. Accordingly, the amount and composition of its tax losses, credits, and other deferred tax assets included in the consolidated and combined financial statements has changed as the result of the Company’s separation from bluebird bio. The components of loss before income taxes were as follows (in thousands): Year ended December 31, 2023 2022 2021 Domestic $ (217,570) $ (254,153) $ (292,213) Foreign — — — Total $ (217,570) $ (254,153) $ (292,213) The Company has not recorded a provision for federal or state income taxes as it has had cumulative net operating losses since inception. For the years ended December 31, 2023, 2022 and 2021, the Company did not recognize any income tax expense (benefit) as the Company was subject to a full valuation allowance. A reconciliation of income tax expense (benefit) computed at the statutory federal income tax rate to the Company’s effective income tax rate as reflected in the financial statements is as follows: Year ended December 31, 2023 2022 2021 Federal income tax expense at statutory rate 21.0 % 21.0 % 21.0 % State income tax, net of federal benefit 6.0 % 6.6 % 0.6 % Permanent differences (0.2) % (0.2) % — % Goodwill (1.2) % — % — % Stock-based compensation (9.2) % (2.5) % 0.3 % Research and development credit 5.3 % 3.2 % 0.2 % Officer compensation limitation 4.0 % (1.2) % (0.4) % Uncertain tax positions (0.4) % (0.2) % — % Other 0.7 % (0.6) % (1.1) % Change in tax rate (1.8) % 1.3 % — % Change in valuation allowance (24.2) % (27.4) % 64.8 % Separation adjustment — % — % (85.4) % Effective income tax rate (expense) benefit — % — % — % Deferred taxes are recognized for temporary differences between the basis of assets and liabilities for financial statement and income tax purposes. The significant components of the Company’s deferred tax assets and liabilities are composed of the following (in thousands): Year ended December 31, 2023 2022 Deferred tax assets: U.S. net operating loss carryforwards (federal and state) $ 38,685 $ 27,251 Tax credit carryforwards (federal and state) 24,933 10,071 Capitalized license fees and research and development expenses 10,562 11,888 Deferred revenue 5,015 3,237 Stock-based compensation 11,834 16,719 Lease liabilities 67,222 73,719 Accruals and other 2,336 3,857 Capitalized research and development expenses 88,721 55,719 Total deferred tax assets 249,308 202,461 Right-of-use assets (57,087) (65,181) Fixed assets (11,499) (9,266) Less: valuation allowance (180,722) (128,014) Net deferred taxes $ — $ — As of December 31, 2023, the Company had U.S. federal net operating loss carryforwards of approximately $143.4 million, which may be available to offset future income tax liabilities and which will carryforward indefinitely. As of December 31, 2023, the Company also had U.S. state net operating loss carryforwards of approximately $139.6 million, which may be available to offset future income tax liabilities and expire at various dates through 2043. As of December 31, 2023, the Company had federal research and development and orphan drug tax credit carryforwards of approximately $19.7 million, available to reduce future tax liabilities which expire at various dates through 2043. As of December 31, 2023, the Company also had U.S. state research and development carryforwards of approximately $8.9 million, which may be available to offset future income tax liabilities and expire at various dates through 2038. An analysis of the U.S. research and development and orphan drug credits has not yet been completed. Until a study is completed by the Company and any limitation is known, no amounts are being presented as an uncertain tax position. Utilization of the net operating loss carryforwards and research and development tax credit carryforwards may be subject to an annual limitation under Section 382 of the Internal Revenue Code of 1986, as amended (the “Internal Revenue Code”), and corresponding provisions of state law, due to ownership changes that have occurred previously or that could occur in the future. These ownership changes may limit the amount of carryforwards that can be utilized annually to offset future taxable income. In general, an ownership change, as defined by Section 382, results from transactions increasing the ownership of certain shareholders or public groups in the stock of a corporation by more than 50 percent over a three-year period. The Company has not conducted a study to assess whether a change of control has occurred or whether there have been multiple changes of control since inception due to the significant complexity and cost associated with such a study. If the Company has experienced a change of control, as defined by Section 382, at any time since inception, utilization of the net operating loss carryforwards or research and development tax credit carryforwards would be subject to an annual limitation under Section 382, which is determined by first multiplying the value of the Company’s stock at the time of the ownership change by the applicable long-term tax-exempt rate, and then could be subject to additional adjustments, as required. Any limitation may result in expiration of a portion of the net operating loss carryforwards or research and development tax credit carryforwards before utilization. Further, until a study is completed and any limitation is known, no amounts are being presented as an uncertain tax position. A valuation allowance is recorded against deferred tax assets if it is more likely than not that some or all of the deferred tax assets will not be realized. Due to the uncertainty surrounding the realization of the favorable tax attributes in future tax returns, the Company has recorded a full valuation allowance against the Company’s otherwise recognizable net deferred tax assets. With respect to the period from separation through December 31, 2023, the valuation allowance increased on a net basis by approximately $52.7 million primarily due to capitalization of R&D expenses and current year losses generated. On December 22, 2017, the Tax Cuts and Jobs Act (“The TCJA”) was signed into law. Under the TCJA provisions, effective with tax years beginning on or after January 1, 2022, taxpayers can no longer immediately expense qualified research and development expenditures, including all direct, indirect, overhead, and software development costs. Taxpayers are now required to capitalize and amortize these costs over five years for research conducted within the United States or 15 years for research conducted abroad. As a result, the Company capitalized $203.4 million of research and development expenses for the year ended December 31, 2023. The Company has no history of tax audits on a standalone basis and will regularly assess the outcome of potential examinations in each of the taxing jurisdictions when determining the adequacy of the amount of unrecognized tax benefit recorded. A reconciliation of the beginning and ending amount of unrecognized tax benefits is as follows (in thousands): Unrecognized tax benefits Balance as of December 31, 2021 36 Increases (decreases) for tax positions related to current period 713 Increases (decreases) for tax positions as part of separation adjustment — Balance as of December 31, 2022 $ 749 Increases (decreases) for tax positions related to current period 1,051 Increases (decreases) for tax positions as part of separation adjustment — Balance as of December 31, 2023 $ 1,800 The unrecognized tax benefits at December 31, 2023 are not material and, if recognized, would not affect the Company’s effective tax rate due to its full valuation allowance position. The Company does not anticipate that the amount of existing unrecognized tax benefits will significantly increase or decrease within the next 12 months. The Company has elected to include interest and penalties related to uncertain tax positions as a component of its provision for income taxes. For the year ended December 31, 2023, the Company’s accrued interest and penalties related to uncertain tax positions were not material.

Net loss per share

Net loss per share	12 Months Ended
Net loss per share	Dec. 31, 2023
Earnings Per Share [Abstract]
Net loss per share	Net loss per share The following common stock equivalents were excluded from the calculation of diluted net loss per share for the periods indicated because including them would have had an anti-dilutive effect (in thousands): Year ended December 31, 2023 2022 2021 Outstanding stock options (1) 3,678 2,562 1,490 Restricted stock units (1) 2,239 1,314 1,080 Employee Stock Purchase Plan 81 35 — 5,998 3,911 2,570 (1)

Corporate restructuring

Corporate restructuring	12 Months Ended
Corporate restructuring	Dec. 31, 2023
Restructuring and Related Activities [Abstract]
Corporate restructuring	Corporate restructuring In August 2023, the Company’s board of directors approved a restructuring plan (the “Restructuring Plan”) to conserve financial resources and better align the Company’s workforce with current business needs. As part of the Restructuring Plan, the Company's workforce was reduced by approximately 40% in September 2023. The Restructuring Plan is substantially complete as of December 31, 2023. In connection with the Restructuring Plan, the Company incurred $8.6 million of one-time costs relating to severance and retention packages and related benefits. These costs were recognized in the third quarter of 2023, in accordance with ASC 420, Exit and Disposal Activities, and are included in Restructuring expenses in the consolidated statement of operations and comprehensive loss. The following table summarizes the accrued liabilities activity recorded in connection with the Restructuring Plan as of December 31, 2023: As of December 31, 2023 Beginning balance $ — Total estimated expenses 8,614 Expenses paid from inception through December 31, 2023 (6,253) Reversal of excess accrual through December 31, 2023 — Remaining accrual at December 31, 2023 (1) $ 2,361 (1)

Goodwill impairment

Goodwill impairment	12 Months Ended
Goodwill impairment	Dec. 31, 2023
Goodwill and Intangible Assets Disclosure [Abstract]
Goodwill impairment	Goodwill impairment On June 30, 2014, bluebird bio acquired Pregenen. All assets and liabilities related to the Pregenen acquisition, including the resulting goodwill and contingent consideration, were attributed to the Company in connection with the separation from bluebird bio. Prior to the impairment test described further below, the balance of the Company’s goodwill was $12.1 million. The Company operates in a single segment, focusing on researching, developing and commercializing potentially transformative treatments for cancer. Consistent with its operational structure, its chief operating decision maker manages and allocates resources for the Company at a consolidated level. Additionally, the Company determined that its single operating segment is also its only reporting unit. As such, the Company has allocated its entire goodwill balance to its single reporting unit and the goodwill impairment test is completed at this level. During the third quarter of 2023 and more recently, the Company experienced a sustained decline in the price of its common stock in part due to decreased external expectations for future Abecma sales resulting from increased competitive dynamics, which was considered a triggering event. Management concluded it was more likely than not that the fair value of its reporting unit is less than its carrying amount. The Company then performed a one-step quantitative test and recorded the amount of goodwill impairment as the excess of the reporting unit’s carrying amount over its fair value, not to exceed the total amount of goodwill allocated to the reporting unit. At September 30, 2023, the Company estimated the fair value of the Company’s single reporting unit using both a market approach and an income approach. Major assumptions were applied in the income approach, including (i) forecasted growth rates (ii) forecasted profitability and (iii) discount rate. Considerable management judgment is necessary to evaluate the impact of operating changes and business initiatives on estimated future growth rates and profitability in order to estimate future cash flows. Upon completing the impairment test, the Company determined that the estimated fair value of the reporting unit was less than its carrying value, thus indicating an impairment. The Company recognized a goodwill impairment charge of $12.1 million during the third quarter of 2023, which represented the entire goodwill balance prior to impairment charge. The following table summarizes the activity of goodwill for the twelve months ended December 31, 2023 (in thousands): Balance at December 31, Additions Deductions Balance at December 31, Goodwill $ 12,056 $ — $ (12,056) $ —

Subsequent events

Subsequent events	12 Months Ended
Subsequent events	Dec. 31, 2023
Subsequent Events [Abstract]
Subsequent events	Subsequent events Regeneron Transaction On January 29, 2024, the Company entered into an asset purchase agreement (the “Purchase Agreement”) with Regeneron. Subject to the terms and conditions of the Purchase Agreement, the Company has agreed to sell to Regeneron (the “Asset Sale”) substantially all of the assets related to its solid tumor and other oncology and autoimmune cell therapy programs (collectively, the “Programs” and such assets, the “Transferred Assets”). Pursuant to the Purchase Agreement, in consideration for the Transferred Assets, at the closing of the Asset Sale (the “Closing”), which is expected in the first half of 2024, Regeneron will make an upfront payment to the Company of $5.0 million in cash and also assume certain liabilities of the Company arising after Closing, including liabilities related to the conduct of the Programs, under transferred contracts and with respect to certain of the Company’s employees (collectively, the “Assumed Liabilities”). Regeneron also agreed to sublease the Company’s facilities in Seattle, Washington and a portion of the Company’s facilities in Cambridge, Massachusetts. In addition to the upfront consideration, Regeneron has agreed to pay the Company a one-time $10.0 million milestone payment upon receipt of regulatory approval for the first product candidate within the Transferred Assets in certain specified countries and agreed-upon royalty payments based on net sales of the product candidates if commercialized. The Purchase Agreement contains customary representations, warranties and covenants of each of the Company and Regeneron. The Purchase Agreement further provides that, subject to certain limitations, the Company and Regeneron will each indemnify the other for certain losses arising from such breaches of representations, warranties and covenants and liabilities allocated to such party pursuant to the terms of the Purchase Agreement. Effective as of the Closing, the Regeneron Collaboration Agreement will terminate. The Closing is subject to customary closing conditions, including, among others: (a) the absence of laws enjoining, making illegal or otherwise prohibiting the Asset Sale; (b) the accuracy of the other party’s representations and warranties, subject to certain customary materiality standards; (c) compliance in all material respects by the other party with its obligations under the Purchase Agreement; and (d) no Material Adverse Effect (as defined in the Purchase Agreement) having occurred with respect to the Transferred Assets and Assumed Liabilities, taken as a whole, since the date of the Purchase Agreement. Upon the Closing, the parties will enter into certain ancillary agreements, including a transition services agreement, a license agreement and sublease agreements for the Company’s facilities as described above. Corporate Restructuring The Board approved a strategic realignment and workforce reduction of approximately 14% on January 29, 2024 in connection with the Asset Sale. The workforce reduction is expected to be substantially complete by the end of the second quarter of 2024. In connection with the workforce reduction and restructuring, the Company expects to incur one-time cash costs of approximately $8.0 million to $10.0 million, primarily in the first half of 2024, relating to severance and retention packages and related benefits. Non-cash expenses associated with the restructuring are not yet estimable. The estimates of non-cash expenses and cash costs that the Company expects to incur, and the timing thereof, are subject to a number of assumptions and actual results may differ. The Company may also incur additional costs not currently contemplated due to events that may occur as a result of, or that are associated with, the actions described above.

Pay vs Performance Disclosure

Pay vs Performance Disclosure - USD ($) $ in Thousands	12 Months Ended
Pay vs Performance Disclosure - USD ($) $ in Thousands	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Pay vs Performance Disclosure
Net loss	$ (217,570)	$ (254,153)	$ (292,213)

Insider Trading Arrangements

Insider Trading Arrangements	12 Months Ended
Insider Trading Arrangements	Dec. 31, 2023
Trading Arrangements, by Individual
Rule 10b5-1 Arrangement Adopted	false
Non-Rule 10b5-1 Arrangement Adopted	false
Rule 10b5-1 Arrangement Terminated	false
Non-Rule 10b5-1 Arrangement Terminated	false

Summary of significant accoun_2

Summary of significant accounting policies and basis of presentation (Policies)	12 Months Ended
	Dec. 31, 2023
Accounting Policies [Abstract]
Basis of presentation and Change in financial statement presentation	Basis of presentation The Company did not operate as a separate, stand-alone entity prior to its separation from bluebird bio. The Company’s consolidated balance sheets as of December 31, 2023 and 2022, statement of operations and comprehensive loss for the years ended December 31, 2023 and 2022, statement of stockholder’s equity for the years ended December 31, 2023 and 2022 and the statement of cash flows for the years ended December 31, 2023 and 2022 consist of the consolidated balances of the Company as prepared on a stand-alone basis. The Company’s consolidated and combined statements of operations and comprehensive loss, stockholders’ equity and cash flows for the year ended December 31, 2021, have been prepared on a carve out basis, derived from bluebird bio’s consolidated financial statements and accounting records, for the period prior to the separation on November 4, 2021 and on a stand-alone basis for the period following the separation through December 31, 2021. The accompanying consolidated and combined financial statements reflect the historical results of the operations, financial position and cash flows of the Company and have been prepared by the Company in accordance with accounting principles generally accepted in the United States (“GAAP”). Any reference in these notes to applicable guidance is meant to refer to the authoritative GAAP as included in the Accounting Standards Codification (“ASC”) and Accounting Standards Updates (“ASUs”) of the Financial Accounting Standards Board (“FASB”). The historical results of operations, financial position and cash flows of 2seventy bio presented in these consolidated and combined financial statements may not be indicative of what they would have been had 2seventy bio been an independent stand-alone entity for each of the years presented prior to the separation, nor are they necessarily indicative of 2seventy bio's future results of operations, financial position and cash flows. Prior to the separation, the Company was dependent upon bluebird bio for all of its working capital and financing requirements, as bluebird bio used a centralized approach to cash management and financing its operations. There were no cash amounts specifically attributable to the Company for the periods prior to the separation; therefore, cash and cash equivalents were not allocated to the Company for periods prior to the separation. Financing transactions related to bluebird bio for periods prior to the separation are accounted for as a component of net parent investment in the consolidated and combined balance sheets and as a financing activity on the accompanying consolidated and combined statements of cash flows. In 2021, net cash transferred to us from bluebird bio of $354.9 million, as indicated in our consolidated and combined statements of cash flows, includes both the net changes in our cash used for operating and investing activities prior to the separation and the cash and cash equivalents distributed to us upon separation. Prior to the separation, the Company’s combined financial statements included an allocation of expenses related to certain bluebird bio corporate functions, including senior management, legal, human resources, finance and information technology. In addition, prior to the separation the Company's combined financial statements include an allocation of certain research and development costs not directly attributable to individual programs. These expenses were allocated to the Company based on direct usage or benefit where specifically identifiable, with the remainder allocated based on employee time spent on projects, square footage or other measures that management believes are consistent and reasonable. These allocations may not be indicative of the actual expense that would have been incurred had the Company operated as an independent, publicly traded company for the periods prior to the separation. See Note 14, Related-party transactions , for a further description of the accounting for the separation from bluebird bio. As 2seventy bio's operations were not historically held by a single legal entity or separate legal entities, net parent investment was shown in lieu of stockholder's equity in the combined financial statements for periods prior to the separation. As a result of the separation, the Company’s net parent investment balance was reclassified to additional paid-in capital. For periods prior to the separation, the taxable income (loss) of bluebird bio entities, including 2seventy bio, Inc. prior to the separation, was included in bluebird bio’s consolidated tax returns. As such, separate income tax returns were not prepared for the entities included within the combined financial statements prior to separation. In connection with the separation, certain assets and liabilities, including certain accounts receivables and accounts payables, included on the consolidated and combined balance sheets prior to the separation have been retained by bluebird bio post-separation and, therefore, were adjusted through net parent investment in the Company’s consolidated and combined financial statements. In addition, in connection with the separation, certain equity awards were converted in accordance with the Employee Matters Agreement, as further described in Note 13, Stock-based compensation . Upon separation, bluebird bio contributed $384.5 million of net assets to the Company. Amounts reported are computed based on thousands, except percentages or as otherwise noted. As a result, certain totals may not sum due to rounding.
Principles of combination	Principles of consolidation and combination Periods prior to separation The accompanying combined financial statements included the attribution of certain assets and liabilities that were historically held by bluebird bio but which were specifically identifiable or attributable to the Company. All intercompany balances and transactions with bluebird bio were deemed to be effectively settled in the combined financial statements at the time the transaction was recorded. Expenses related to corporate allocations from bluebird bio to the Company were considered to be effectively settled for cash in the combined financial statements at the time the transaction was recorded. Periods after the separation The accompanying consolidated and combined financial statements include the accounts of the Company and its wholly owned subsidiaries. All intercompany transactions and accounts within the Company have been eliminated. Certain amounts presented in the prior period have been reclassified to conform to the current period presentation. As a standalone entity, the Company has filed tax returns on its own behalf, and tax balances and the effective income tax rate may have differed from the amounts reported in the historical periods. As of November 4, 2021 and in connection with the separation, the Company adjusted its deferred tax balances and computed its related tax provision to reflect operations as a standalone entity. Variable interest entities The Company continually assesses whether it is the primary beneficiary of a variable interest entity as changes to existing relationships or future transactions may result in consolidation or deconsolidation of one or more collaborators or partners. In determining whether it is the primary beneficiary of an entity in which the Company has a variable interest, management applies a qualitative approach that determines whether the Company has both (1) the power to direct the economically significant activities of the entity and (2) the obligation to absorb losses of, or the right to receive benefits from, the entity that could potentially be significant to that entity.
Use of estimates	Use of estimates The preparation of financial statements in conformity with GAAP requires management to make estimates and assumptions that affect the reported amounts in the financial statements and accompanying notes. Actual results could materially differ from those estimates. Management considers many factors in selecting appropriate financial accounting policies and controls, and in developing the estimates and assumptions that are used in the preparation of these financial statements. Management must apply significant judgment in this process. In addition, other factors may affect estimates, including: expected business and operational changes, sensitivity and volatility associated with the assumptions used in developing estimates, and whether historical trends are expected to be representative of future trends. The estimation process often may yield a range of potentially reasonable estimates of the ultimate future outcomes and management must select an amount that falls within that range of reasonable estimates. This process may result in actual results differing materially from those estimated amounts used in the preparation of the financial statements. Estimates and judgments are used in the following areas, among others: allocations of revenue, expenses, assets and liabilities from bluebird bio's historical consolidated financial statements to the Company for periods prior to the separation, future undiscounted cash flows and subsequent fair value estimates used to assess potential and measure any impairment of long-lived assets, including goodwill and intangible assets, the measurement of right-of-use assets and lease liabilities, contingent consideration, stock-based compensation expense, accrued expenses, income taxes, and the assessment of the Company's ability to fund its operations for at least the next twelve months from the date of issuance of these financial statements. In addition, estimates and judgments are used in the Company’s accounting for its revenue-generating arrangements, in particular as it relates to determining the stand-alone selling price of performance obligations, evaluating whether an option to acquire additional goods and services represents a material right, estimating the total transaction price, including estimating variable consideration and the probability of achieving future potential development and regulatory milestones, assessing the period of performance over which revenue may be recognized, and accounting for modifications to revenue-generating arrangements.
Segment information	Segment information The Company operates in a single segment, focusing on researching, developing and commercializing potentially transformative treatments for cancer. Consistent with its operational structure, its chief operating decision maker manages and allocates resources for the Company at a consolidated level. Therefore, results of the Company's operations are reported on a consolidated basis for purposes of segment reporting. All material long-lived assets of the Company reside in the United States.
Cash and cash equivalents	Cash and cash equivalents The Company considers all highly liquid investments purchased with original final maturities of 90 days or less from the date of purchase to be cash equivalents. Cash equivalents may consist of marketable securities with maturities of less than 90 days when purchased. Cash equivalents are reported at fair value. There were no cash or cash equivalents specifically attributable to 2seventy bio for the historical periods presented prior to the separation.
Restricted investments	Restricted investments As of December 31, 2023 and 2022, restricted investments consists primarily of United States government agency securities and treasuries placed as collateral to letters of credit totaling $32.8 million and $30.0 million, respectively that are required to be maintained in connection with the Company’s lease arrangements. The letters of credit are in the name of the Company’s landlord and are required to fulfill lease requirements in the event the Company should default on its lease obligations. As of December 31, 2023 and 2022, the Company classified its restricted investments as non-current on the consolidated balance sheet based on the release dates of the restrictions. For further information on the Company’s letters of credit, see Note 7, Leases .
Marketable securities	Marketable securities The Company’s marketable securities are maintained by investment managers and consist of U.S. government agency securities and treasuries, corporate bonds and commercial paper. Debt securities are carried at fair value with the unrealized gains and losses included in other comprehensive income (loss) as a component of stockholders’ equity until realized. Any premium arising at purchase is amortized to the earliest call date and any discount arising at purchase is accreted to maturity. Amortization and accretion of premiums and discounts are recorded in interest income, net. Realized gains and losses on debt securities are determined using the specific identification method and are included in other income, net. The Company classifies marketable securities with a remaining maturity when purchased of greater than three months as available-for-sale. Marketable securities with a remaining maturity date greater than one year are classified as non-current assets although these securities are available for use in the Company’s current operations. There were no marketable securities specifically attributable to 2seventy bio for the historical periods presented prior to the separation. The Company assesses its available-for-sale debt securities under the available-for-sale debt security impairment model in ASU 2016-13, Financial Instruments - Credit Losses (Topic 326): Measurement of Credit Losses on Financial Statements (“ASU 2016-13” or “ASC 326”) as of each reporting date in order to determine if a portion of any decline in fair value below carrying value recognized on its available-for-sale debt securities is the result of a credit loss. The Company records credit losses in the consolidated and combined statements of operations and comprehensive loss as credit loss expense within other income, net, which is limited to the difference between the fair value and the amortized cost of the security. To date, the Company has not recorded any credit losses on its available-for-sale debt securities. Accrued interest receivable related to the Company's available-for-sale debt securities is presented within receivables and other current assets on the Company's consolidated balance sheets. The Company has elected the practical expedient available to exclude accrued interest receivable from both the fair value and the amortized cost basis of available-for-sale debt securities for the purposes of identifying and measuring any impairment. The Company writes off accrued interest receivable once it has determined that the asset is not realizable. Any write offs of accrued interest receivable are recorded by reversing interest income, recognizing credit loss expense, or a combination of both. To date, the Company has not written off any accrued interest receivables associated with its marketable securities.
Concentrations of credit risk and off-balance sheet risk	Concentrations of credit risk and off-balance sheet risk Financial instruments that subject the Company to credit risk primarily consist of cash and cash equivalents, available-for-sale securities, and accounts receivable. Deposits are maintained with banks in the United States and such amounts may exceed the amount of insurance provided on such deposits. The Company has not experienced any losses on such accounts, and does not believe it is exposed to any unusual credit risk beyond the normal credit risk currently associated with commercial banking relationships. Cash deposits are insured by the Federal Deposit Insurance Corporations (“FDIC”) up to $250,000. The Company has cash deposits in excess of the FDIC insured limit. The Company’s marketable securities, which primarily consist of U.S. government agency securities and treasuries, corporate bonds and commercial paper, potentially subject the Company to concentrations of credit risk. The Company has adopted an investment policy that limits the amounts the Company may invest in any one type of investment and requires all investments held by the Company to be at least AA+/Aa1 rated, thereby reducing credit risk exposure. The Company’s accounts receivable balance primarily consists of amounts owed from collaborative arrangement partners and royalties receivable under out-license agreements with various third parties. The Company monitors economic conditions to identify facts or circumstances that may indicate that any of its accounts receivable are at risk of collection.
Fair value of financial instruments	Fair value of financial instruments The Company has certain financial assets and liabilities recorded at fair value which have been classified as Level 1, 2 or 3 within the fair value hierarchy as described in the accounting standards for fair value measurements: Level 1—Fair values are determined utilizing quoted prices (unadjusted) in active markets for identical assets or liabilities that the Company has the ability to access at the measurement date. Level 2—Fair values are determined utilizing quoted prices for identical assets or liabilities in non-active markets, quoted prices for similar assets or liabilities in active markets, or other market observable inputs such as interest rates, yield curves and foreign currency spot rates. Level 3—Prices or valuations that require inputs that are both significant to the fair value measurement and unobservable. To the extent that valuation is based on models or inputs that are less observable or unobservable in the market, the determination of fair value requires more judgment. Accordingly, the degree of judgment exercised by the Company in determining fair value is greatest for instruments categorized in Level 3. A financial instrument’s level within the fair value hierarchy is based on the lowest level of any input that is significant to the fair value measurement. Items measured at fair value on a recurring basis include marketable securities (see Note 3, Marketable securities , and Note 4, Fair value measurements ) and restricted cash and cash equivalents, restricted investments, and contingent consideration (see Note 4, Fair value measurements ). The carrying amounts of accounts receivable, accounts payable and accrued expenses approximate their fair values due to their short-term nature.
Business combinations	Business combinations Business combinations are accounted for using the acquisition method of accounting. Using this method, the tangible and intangible assets acquired and the liabilities assumed are recorded as of the acquisition date at their respective fair values. The Company evaluates a business as an integrated set of activities and assets that is capable of being managed for the purpose of providing a return in the form of dividends, lower costs or other economic benefits and consists of inputs and processes that provide or have the ability to provide outputs. In an acquisition of a business, the excess of the fair value of the consideration transferred over the fair value of the net assets acquired is recorded as goodwill. In an acquisition of net assets that does not constitute a business, no goodwill is recognized.
Goodwill	Goodwill Goodwill represents the excess of the purchase price over the fair value of the net assets acquired when accounted for using the acquisition method of accounting. Goodwill is not amortized; rather, it is evaluated for impairment within the Company’s single reporting unit on an annual basis, during the fourth quarter, or more frequently if an event occurs or circumstances change that would more-likely-than-not reduce the fair value of the Company’s reporting unit below its carrying amount. The Company performs a one-step quantitative test and records the amount of goodwill impairment, if any, as the excess of a reporting unit's carrying amount over its fair value, not to exceed the total amount of goodwill allocated to the reporting unit. As noted within “Change in Fair Value of Contingent Consideration” above, the Company assumed goodwill related to the Pregenen acquisition upon its separation from bluebird bio. As discussed further in Note 17, Goodwill , the sustained decline in the price of the Company’s common stock in part due to decreased external expectations for future Abecma
Intangible assets, net	Intangible assets, net Intangible assets, net primarily consist of acquired core technology, net of accumulated amortization. The Company amortizes its intangible assets using the straight-line method over their estimated economic lives and periodically reviews for impairment. The Company has not recognized any impairment charges related to intangible assets to date.
Contingent consideration	Contingent consideration Each reporting period, the Company remeasures the contingent consideration obligations associated with business combinations to their fair value and records within operating expenses increases or decreases in their fair value as change in fair value of contingent consideration within the consolidated and combined statements of operations and comprehensive loss. Changes in contingent consideration result from changes in the assumptions regarding probabilities of successful achievement of related milestones, the estimated timing in which the milestones may be achieved, and the discount rate used to estimate the fair value of the liability. Contingent consideration may change significantly as development of the Company’s programs in certain indications progress and additional data is obtained, impacting the Company’s assumptions. The assumptions used in estimating fair value require significant judgment. The use of different assumptions and judgments could result in a materially different estimate of fair value. See Note 4, Fair value measurements, for additional information.
Property, plant and equipment	Property, plant and equipment Property, plant and equipment is stated at cost. Maintenance and repairs that do not improve or extend the lives of the respective assets are expensed to operations as incurred. Depreciation and amortization is calculated using the straight-line method over the estimated useful lives of the assets, which are as follows: Asset Estimated useful life Building 40 years Computer equipment and software 3 years Furniture and fixtures 2-5 years Laboratory equipment 5-10 years Leasehold improvements Shorter of the useful life or remaining lease term
Impairment of long-lived assets	Impairment of long-lived assets The Company reviews long-lived assets when events or changes in circumstances indicate the carrying value of the assets may not be recoverable. Recoverability is measured by comparison of the book values of the assets to future net undiscounted cash flows that the assets are expected to generate. If such assets are considered to be impaired, the impairment to be recognized is measured by the amount by which the book value of the assets exceed their fair value, which is measured based on the projected discounted future net cash flows arising from the assets.
Leases	Leases At the inception of an arrangement, the Company determines whether the arrangement is or contains a lease based on the relevant facts and circumstances present in the arrangement. Leases with a term greater than one year are recognized on the balance sheet as right-of-use assets and current and non-current lease liabilities, as applicable. The Company does not have material financing leases. Operating lease liabilities and their corresponding right-of-use assets are initially recorded based on the present value of lease payments over the expected remaining lease term. Certain adjustments to the right-of-use asset may be required for items such as incentives received. The interest rate implicit in lease contracts is typically not readily determinable. As a result, the Company utilizes its incremental borrowing rate to discount lease payments, which reflects the rate at which the Company could borrow on a collateralized basis the amount of the lease payments in the same currency, for a similar term, in a similar economic environment. To estimate its incremental borrowing rate, a credit rating applicable to the Company is estimated using a synthetic credit rating analysis since the Company does not currently have a rating agency-based credit rating. The Company has elected not to recognize leases with an original term of one year or less on the balance sheet. The Company typically only includes an initial lease term in its assessment of a lease arrangement. Options to renew a lease are not included in the Company’s assessment unless it is reasonably certain that the Company will exercise its renewal option. Assumptions made by the Company at the commencement date are re-evaluated upon occurrence of certain events, including a lease modification. A lease modification results in a separate contract when the modification grants the lessee an additional right of use not included in the original lease and when lease payments increase commensurate with the stand-alone price for the additional right of use. When a lease modification results in a separate contract, it is accounted for in the same manner as a new lease. In accordance with ASC 842, components of a lease should be separated into lease components and non-lease components. The fixed and in-substance fixed contract consideration must be allocated based on the relative stand-alone prices to the lease components and non-lease components. Entities may elect not to separate lease and non-lease components. Rather, entities would account for each lease component and related non-lease component together as a single lease component. The Company has elected to account for lease and non-lease components together as a single lease component for all underlying assets and allocate all of the contract consideration to the lease component only. For purposes of distinguishing between operating and financing leases, ASC 842 allows for the use of judgment in determining whether the lease term is for a major part of the remaining economic life of the underlying asset and whether the present value of lease payments represents substantially all of the fair value of the underlying asset. The Company applies the bright line thresholds referenced in ASC 842-10-55-2 to assist in evaluating leases for appropriate classification. The aforementioned bright lines are applied consistently to the Company’s entire portfolio of leases.
Common stock warrants	Common stock warrants The Company's common stock warrants are evaluated pursuant to ASC 480, Distinguishing Liabilities from Equity ("ASC 480"), and ASC 815, Derivatives and Hedging ("ASC 815"). Management classifies its freestanding warrants as (i) liabilities, if the warrant terms allow settlement of the warrant exercise in cash, or (ii) equity, if the warrant terms only allow settlement in shares of common stock.
Revenue recognition	Revenue recognition Under ASC Topic 606, Revenue from Contracts with Customers (“Topic 606”), an entity recognizes revenue when its customer obtains control of promised goods or services, in an amount that reflects the consideration that the entity expects to receive in exchange for those goods or services. To determine revenue recognition for arrangements that an entity determines are within the scope of Topic 606, the entity performs the following five steps: (i) identify the contract(s) with a customer; (ii) identify the performance obligations in the contract; (iii) determine the transaction price, including variable consideration, if any; (iv) allocate the transaction price to the performance obligations in the contract; and (v) recognize revenue when (or as) the entity satisfies a performance obligation. The Company only applies the five-step model to contracts when it is probable that the entity will collect the consideration to which it is entitled in exchange for the goods or services it transfers to the customer. Once a contract is determined to be within the scope of Topic 606, the Company assesses the goods or services promised within each contract and determines those that are performance obligations. Arrangements that include rights to additional goods or services that are exercisable at a customer’s discretion are generally considered options. The Company assesses if these options provide a material right to the customer and if so, they are considered performance obligations. The identification of material rights requires judgments related to the determination of the value of the underlying good or service relative to the option exercise price. The exercise of a material right is accounted for as a contract modification for accounting purposes. The Company assesses whether each promised good or service is distinct for the purpose of identifying the performance obligations in the contract. This assessment involves subjective determinations and requires management to make judgments about the individual promised goods or services and whether such are separable from the other aspects of the contractual relationship. Promised goods and services are considered distinct provided that: (i) the customer can benefit from the good or service either on its own or together with other resources that are readily available to the customer (that is, the good or service is capable of being distinct) and (ii) the entity’s promise to transfer the good or service to the customer is separately identifiable from other promises in the contract (that is, the promise to transfer the good or service is distinct within the context of the contract). In assessing whether a promised good or service is distinct, the Company considers factors such as the license terms, the research, manufacturing and commercialization capabilities of the collaboration partner and the availability of the associated expertise in the general marketplace. The Company also considers the intended benefit of the contract in assessing whether a promised good or service is separately identifiable from other promises in the contract. If a promised good or service is not distinct, an entity is required to combine that good or service with other promised goods or services until it identifies a bundle of goods or services that is distinct. The transaction price is determined and allocated to the identified performance obligations in proportion to their stand-alone selling prices (“SSP”) on a relative SSP basis. SSP is determined at contract inception and is not updated to reflect changes between contract inception and when the performance obligations are satisfied. Determining the SSP for performance obligations requires significant judgment. In developing the SSP for a performance obligation, the Company considers applicable market conditions and relevant entity-specific factors, including factors that were contemplated in negotiating the agreement with the customer and estimated costs. The Company validates the SSP for performance obligations by evaluating whether changes in the key assumptions used to determine the SSP will have a significant effect on the allocation of arrangement consideration between multiple performance obligations. If the consideration promised in a contract includes a variable amount, the Company estimates the amount of consideration to which it will be entitled in exchange for transferring the promised goods or services to a customer. The Company determines the amount of variable consideration by using the expected value method or the most likely amount method. The amount of variable consideration included in the transaction price is constrained to the amount for which it is probable that a significant reversal of cumulative revenue recognized will not occur when the uncertainty related to the variable consideration is resolved. At the end of each subsequent reporting period, the Company re-evaluates the estimated variable consideration included in the transaction price and any related constraint, and if necessary, adjusts its estimate of the overall transaction price. Any such adjustments are recorded on a cumulative catch-up basis in the period of adjustment. If an arrangement includes development and regulatory milestone payments, the Company evaluates whether the milestones are considered probable of being reached and estimates the amount to be included in the transaction price using the most likely amount method. If it is probable that a significant revenue reversal would not occur, the associated milestone value is included in the transaction price. Milestone payments that are not within the Company’s control or the licensee’s control, such as regulatory approvals, are generally not considered probable of being achieved until those approvals are received. For arrangements with licenses of intellectual property that include sales-based royalties, including milestone payments based on the level of sales, and the license is deemed to be the predominant item to which the royalties relate, the Company recognizes royalty revenue and sales-based milestones at the later of (i) when the related sales occur, or (ii) when the performance obligation to which the royalty has been allocated has been satisfied. In determining the transaction price, the Company adjusts consideration for the effects of the time value of money if the timing of payments provides the Company with a significant benefit of financing. The Company does not assess whether a contract has a significant financing component if the expectation at contract inception is such that the period between payment by the licensees and the transfer of the promised goods or services to the licensees will be one year or less. The Company assessed each of its revenue generating arrangements in order to determine whether a significant financing component exists and concluded that a significant financing component does not exist in any of its arrangements. The Company recognizes as revenue the amount of the transaction price that is allocated to the respective performance obligation when (or as) each performance obligation is satisfied, either at a point in time or over time, and if over time recognition is based on the use of an output or input method. The Company recognizes revenue within the following financial statement captions: Service revenue To date, the Company’s service revenue has primarily been generated from the elements of its collaboration arrangements with BMS and Novo Nordisk that are accounted for pursuant to Topic 606, using the five-step model described above. As discussed further below, the Company analyzes its collaboration arrangements to assess whether they are within the scope of ASC 808, Collaborative Arrangements (“ASC 808”) or Topic 606. For the elements of a collaboration arrangement which are more reflective of a vendor-customer relationship and therefore within the scope of Topic 606, the Company records the related revenue as service revenue on the consolidated and combined statement of operations and comprehensive loss. Refer below for additional discussion around the Company’s policy for recognizing collaborative arrangement revenue and the determination of whether elements of a collaboration arrangement are within the scope of ASC 808 or Topic 606. For the years ending 2023, 2022 and 2021, service revenue consisted of the following: Year Ended December 31, 2023 2022 2021 ide-cel ex-U.S. service revenue from BMS $ 14,751 $ 13,226 $ 16,895 bb21717 license and manufacturing service revenue from BMS — 35,762 — Service revenue from December 2021 agreement with Novo Nordisk 5,765 6,501 — Other 3,628 — 4,486 Total service revenue $ 24,144 $ 55,489 $ 21,381 Collaborative arrangement revenue To date, the Company’s collaborative arrangement revenue has been generated from its collaboration arrangements with BMS and Regeneron, as further described in Note 10, Collaborative arrangements and strategic partnerships. For the years ending 2023, 2022 and 2021, collaborative arrangement revenue consisted of the following: Year Ended December 31, 2023 2022 2021 U.S. Abecma collaboration with BMS $ 50,010 $ 12,781 $ 19,425 Collaboration with Regeneron 21,591 19,577 7,496 Total collaborative arrangement revenue $ 71,601 $ 32,358 $ 26,921 The Company analyzes its collaboration arrangements to assess whether they are within the scope of ASC 808, which includes determining whether such arrangements involve joint operating activities performed by parties that are both active participants in the activities and exposed to significant risks and rewards dependent on the commercial success of such activities. This assessment is performed throughout the life of the arrangement based on changes in the responsibilities of all parties in the arrangement. For collaboration arrangements within the scope of ASC 808 that contain multiple elements, the Company first determines which elements of the collaboration are deemed to be within the scope of ASC 808 and those that are more reflective of a vendor-customer relationship and therefore within the scope of Topic 606 (refer above for further discussion of the Company's policy for recognizing service revenue pursuant to Topic 606). For elements of collaboration arrangements that are accounted for pursuant to ASC 808, an appropriate recognition method is determined and applied consistently, generally by analogy to Topic 606. In arrangements where the Company does not deem its collaborator to be its customer, payments to and from its collaborator are presented in the consolidated and combined statements of operations and comprehensive loss based on the nature of the payments, as summarized in the table and further described below. Nature of Payment Statement of Operations Presentation The Company's share of net profits in connection with commercialization of products Collaborative arrangement revenue The Company's share of net losses in connection with commercialization of products Share of collaboration loss Net reimbursement to the Company for research and development expenses Collaborative arrangement revenue Net reimbursement to the collaborator for research and development expenses Research and development expense Where the collaborator is the principal in the product sales, the Company recognizes its share of any profits or losses, representing net product sales less cost of goods sold and shared commercial and other expenses, in the period in which such underlying sales occur and costs are incurred by the collaborator. The Company also recognizes its share of costs arising from research and development activities performed by collaborators in the period its collaborators incur such expenses. Royalty and other revenue The Company enters into out-licensing agreements that are within the scope of Topic 606. The Company does not have any material license arrangements that contain more than one performance obligation. The terms of such out-license agreements include the license of functional intellectual property, given the functionality of the intellectual property is not expected to change substantially as a result of the licensor’s ongoing activities, and typically include payment of one or more of the following: non-refundable up-front license fees; development and regulatory milestone payments and milestone payments based on the level of sales; and royalties on net sales of licensed products. Nonrefundable up-front license fees are recognized as revenue at a point in time when the licensed intellectual property is made available for the customer’s use and benefit, which is generally at the inception of the arrangement. Development and regulatory milestone fees, which are a type of variable consideration, are recognized as revenue to the extent that it is probable that a significant reversal will not occur. The Company recognizes royalty revenue and sales-based milestones at the later of (i) when the related sales occur, or (ii) when the performance obligation to which the royalty has been allocated has been satisfied. For a complete discussion of accounting for collaboration and other revenue-generating arrangements, see Note 10, Collaborative arrangements and strategic partnerships , and Note 11, Royalty and other revenue.
Research and development expenses	Research and development expenses Research and development costs are charged to expense as costs are incurred in performing research and development activities, including salaries and benefits, facilities costs, overhead costs, clinical study and related clinical manufacturing costs, license and milestone fees, contract services, manufacturing costs for pre-launch inventory that did not qualify for capitalization, and other related costs. Up-front fees and milestones paid to third parties in connection with technologies that have not reached technological feasibility and do not have an alternative future use are expensed as research and development expense as incurred. In circumstances where amounts have been paid in excess of costs incurred, the Company records a prepaid expense. The Company accrues costs for clinical trial activities based upon estimates of the services received and related expenses incurred that have yet to be invoiced by the contract research organizations, clinical study sites, laboratories, consultants, or other clinical trial vendors that perform the activities. Where amounts owed to a collaboration partner exceed the Company’s collaborative arrangement revenues in each quarterly period, such amounts are classified as research and development expense.
Cost of Manufacturing for Commercial Collaboration	Cost of Manufacturing for Commercial Collaboration Cost of manufacturing for commercial collaboration consists of quality and other manufacturing costs incurred by the Company to support the manufacture of Abecma inventory sold by its collaborative partner, BMS, in both the U.S. and in ex-U.S. regions. These costs are subject to the cost sharing arrangement under the terms of the Company’s collaboration agreement (the Amended Ide-cel CCPS) with BMS. For further information on the Amended Ide-cel CCPS, please refer to Note 10, Collaborative arrangements and strategic partnerships . The reimbursement from BMS for their share of the Company’s U.S. quality and other manufacturing costs is recorded as collaborative arrangement revenue or share of collaboration loss in the consolidated and combined statements of operations and comprehensive loss. The reimbursement from BMS for the Company’s ex-U.S. quality and other manufacturing costs is recorded as service revenue in the consolidated and combined statements of operations and comprehensive loss.
Restructuring expenses	Restructuring expenses In September 2023, the Company announced its Restructuring Plan to conserve financial resources and better align its workforce with current business needs. As part of the Restructuring Plan, the Company’s workforce was reduced by approximately 40%, with substantially all of the reduction in personnel completed by December 31, 2023. In connection with the Restructuring Plan, the Company incurred one-time costs in the third quarter of 2023 relating to severance and retention packages and related benefits. These costs were recorded as restructuring expenses in the consolidated statements of operations and comprehensive loss.
Selling, General and Administrative Expenses	Selling, General and Administrative Expenses Selling, general and administrative expenses consist primarily of salaries and related costs for personnel, including stock-based compensation and travel expenses for the Company’s employees in executive, operational, finance, legal, business development, commercial, information technology, and human resource functions. Other selling, general and administrative expenses include facility-related costs, insurance, IT costs, professional fees for accounting, tax, legal and consulting services, directors’ fees and expenses associated with obtaining and maintaining patents.
Share of Collaboration Loss	Share of Collaboration Loss Share of collaboration loss represents the Company’s share of net loss arising from product sales less cost of goods sold and shared commercial costs and other expenses related to the commercialization of a product where the collaborator is the principal in the product sales. For a complete discussion of accounting for share of collaboration loss, see Note 10, Collaborative arrangements and strategic partnerships.
Cost of royalty and other revenue	Cost of royalty and other revenue Cost of royalty and other revenue represents expense associated with amounts owed to third parties as a result of revenue recognized under the Company’s out-license arrangements.
Stock-based compensation	Stock-based compensation The Company’s share-based compensation programs grant awards that include stock options, restricted stock units, restricted stock awards, and shares issued under an employee stock purchase plan. Grants are awarded to employees and non-employees, including the Company's board of directors. The Company accounts for its stock-based compensation awards in accordance with FASB ASC Topic 718, Compensation—Stock Compensation (“ASC 718”). ASC 718 requires all stock-based payments, including grants of stock options and restricted stock units and modifications to existing stock options, to be recognized in the consolidated and combined statements of operations and comprehensive loss based on their fair values. The Company’s stock-based awards are subject to either service, performance-based, or market-based vesting conditions. Compensation expense related to awards with service-based vesting conditions is recognized on a straight-line basis based on the grant date fair value over the associated service period of the award, which is generally the vesting term. Compensation expense related to awards with performance-based vesting conditions is recognized based on the grant date fair value over the requisite service period using the accelerated attribution method to the extent achievement of the performance condition is probable. The Company estimates the fair value of its option awards using the Black-Scholes option pricing model, which requires the input of subjective assumptions, including (i) the expected stock price volatility, (ii) the calculation of expected term of the award, (iii) the risk-free interest rate, and (iv) expected dividends. Due to the lack of company specific historical and implied volatility data, the Company bases its estimate of expected volatility on the estimate and expected volatilities of a representative group of publicly traded companies. For these analyses, the Company selects companies with comparable characteristics including enterprise value, risk profiles, position within the industry, and with historical share price information sufficient to meet the expected life of the stock-based awards. It will continue to apply this process until sufficient historical information regarding the volatility of its own stock price becomes available. The Company has estimated the expected term of its employee stock options using the “simplified” method, whereby, the expected term equals the arithmetic average of the vesting term and the original contractual term of the option due to its lack of sufficient historical data. For stock options that were converted in accordance with the Employee Matters Agreement, as further described in Note 13, Stock-based compensation , the Company estimated the expected term to be the remaining contractual term of the awards as of the date of the separation. The risk-free interest rates for periods within the expected term of the option are based on the U.S. Treasury securities with a maturity date commensurate with the expected term of the associated award. The Company has never paid, and does not expect to pay, dividends in the foreseeable future. For performance-based restricted stock units that vest based on a total shareholder return metric, the vesting condition is considered a market condition as it vests based on the movement in share price of a company. The impact of the market condition is reflected in the award’s fair value on the grant date, and the expense is recognized irrespective of whether the market condition is achieved as long as the required service is provided. The Company accounts for forfeitures as they occur. Stock-based compensation expense recognized in the financial statements is based on awards for which performance or service conditions are expected to be satisfied.
Other income, net	Other income, net For periods prior to the separation, other income, net consisted primarily of income resulting from the allocation of facility-related, depreciation and amortization expense to bluebird bio for its proportional use of assets that were attributed to the Company as well as expense resulting from the allocation of facility-related, depreciation and amortization expense to the Company for its proportional use of bluebird bio assets that were not attributed to the Company. Other income, net also includes immaterial gains and losses on disposal of assets. Following the separation, other income, net primarily consisted of rental income from third parties, income recognized under the Company’s transition services agreements with bluebird bio and sublease income.
Net loss per share	Net loss per share Basic net loss per share is calculated by dividing net loss attributable to common stockholders by the weighted average number of common shares outstanding during the period, including the shares of common stock issuable upon the exercise of warrants that are exercisable for little or no consideration. Diluted net income per share is calculated by dividing the net income attributable to common stockholders by the weighted-average number of common equivalent shares outstanding for the period, including the shares of common stock issuable upon the exercise of warrants that are exercisable for little or no consideration as well as any dilutive effect from outstanding stock options, unvested restricted stock, restricted stock units, and employee stock purchase plan stock using the treasury stock method. Given that the Company recorded a net loss for each of the periods presented, there is no difference between basic and diluted net loss per share since the effect of common stock equivalents would be anti-dilutive and are, therefore, excluded from the diluted net loss per share calculation. The Company follows the two-class method when computing net loss per share in periods when issued shares that meet the definition of participating securities are outstanding. The two-class method determines net loss per share for each class of common and participating securities according to dividends declared or accumulated and participation rights in undistributed earnings. The two-class method requires income available to common stockholders for the period to be allocated between common and participating securities based upon their respective rights to received dividends as if all income for the period had been distributed. Accordingly, in periods in which the Company reports a net loss attributable to common stockholders when participating securities are outstanding, losses are not allocated to the participating securities.
Income taxes	Income taxes For periods prior to the separation, income taxes as presented in the combined financial statements of 2seventy bio attribute current and deferred income taxes of bluebird bio to 2seventy bio in a manner that is systematic, rational and consistent with the asset and liability method prescribed by FASB ASC Topic 740: Income Taxes (“ASC 740”). Accordingly, 2seventy bio’s income tax provision for periods prior to the separation was prepared following the separate return method. The separate return method applies ASC 740 to the stand-alone financial statements of each member of the consolidated group as if each group member was a separate taxpayer and a stand-alone enterprise. The calculation of the Company's income taxes on a separate return basis requires a considerable amount of judgment and use of both estimates and allocations. As a result, actual transactions included in the consolidated financial statements of bluebird bio may not be included in the separate combined financial statements of 2seventy bio. Similarly, the tax treatment of certain items reflected in the combined financial statements of 2seventy bio may not be reflected in the consolidated financial statements and tax returns of bluebird bio. Therefore, items such as net operating losses, credit carryforwards and valuation allowances may exist in the Company's stand-alone financial statements that may or may not exist in bluebird bio’s consolidated financial statements. As such, the income taxes of 2seventy bio as presented for periods prior to the separation may not be indicative of the income taxes that 2seventy bio will generate in the future. The Company recognizes deferred tax assets and liabilities for the expected future tax consequences of events that have been included in the financial statements or tax returns. Deferred tax assets and liabilities are determined based on the difference between the financial statement and tax bases of assets and liabilities using enacted tax rates in effect for the year in which the differences are expected to reverse. Valuation allowances are provided, if based upon the weight of available evidence, it is more likely than not that some or all of the deferred tax assets will not be realized. The Company accounts for uncertain tax positions in accordance with the provisions of ASC 740. When uncertain tax positions exist, the Company recognizes the tax benefit of tax positions to the extent that the benefit will more likely than not be realized. The determination as to whether the tax benefit will more likely than not be realized is based upon the technical merits of the tax position as well as consideration of the available facts and circumstances. The Company accrues for potential interest and penalties related to unrecognized tax benefits in income tax expense. In general, for periods prior to the separation, the taxable income (loss) of bluebird bio, including any taxable income (loss) attributed to 2seventy bio, was included in bluebird bio’s consolidated tax returns. As such, separate income tax returns were not prepared for 2seventy bio prior to the separation. Consequently, for periods prior to the separation any income taxes currently payable by 2seventy bio are deemed to have been remitted to bluebird bio, in cash, in the period prior to the separation in which the liability arose, and income taxes currently receivable by 2seventy bio are deemed to have been received from bluebird bio in the period in which the receivable arose.
Comprehensive loss	Comprehensive loss Comprehensive loss is composed of net loss and other comprehensive income (loss). Other comprehensive income (loss) consists of unrealized gains and losses on debt securities.
Recent accounting pronouncements	Recent accounting pronouncements From time to time, new accounting pronouncements are issued by the Financial Accounting Standards Board (“FASB”) or other standard setting bodies that are adopted by the Company as of the specified effective date. During the years ended December 31, 2023, 2022, and 2021, the Company did not adopt any new accounting pronouncements that had a material effect on its consolidated and combined financial statements. Not yet adopted ASU No. 2023-07, Segment Reporting (Topic 280): Improvements to Reportable Segment Disclosure In November 2023, the FASB issued ASU 2023-07, Segment Reporting (Topic 280: Improvements to Reportable Segment Disclosures) (“ASU 2023-07”). The amendments in this update improve reportable segment disclosure requirements through enhanced disclosures about significant segment expenses. All disclosure requirements of the ASU are required for entities with a single reportable segment. The amendments are effective for fiscal years beginning after December 15, 2023, and interim periods within fiscal years beginning after December 15, 2024, and should be applied on a retrospective basis to all periods presented. The Company is currently evaluating the impact of future adoption on its financial statements and related disclosures. ASU No. 2023-09, Income Taxes (Topic 740): Improvements to Income Tax Disclosures In December 2023, the FASB issued ASU 2023-09, Income Taxes (Topic 740): Improvements to Income Tax Disclosures (“ASU 2023-09”), which is intended to enhance the transparency and decision usefulness of income tax disclosures. This new standard is effective for annual periods beginning after December 15, 2024. The Company plans to adopt this standard following the effective date and the Company does not expect ASU 2023-09 to have a material impact on the Company's financial position or results of operations upon adoption.

Summary of significant accoun_3

Summary of significant accounting policies and basis of presentation (Tables)	12 Months Ended
	Dec. 31, 2023
Accounting Policies [Abstract]
Schedule of Error Corrections and Prior Period Adjustments	adjusted its previously filed consolidated statement of cash flows as follows: For the year ended December 31, 2022 in thousands As previously reported Adjustment As revised Cash flows from operating activities: Changes in operating assets and liabilities: Prepaid expenses and other assets $ (8,356) $ 1,024 $ (7,332) Accrued expenses and other liabilities $ (8,110) $ 7,970 $ (140) Net cash used in operating activities $ (237,134) $ 8,994 $ (228,140) Cash flows from investing activities: Purchases of property, plant and equipment $ (22,842) $ (7,968) $ (30,810) Purchases of restricted investments $ — $ (1,976) $ (1,976) Maturities of restricted investments $ — $ 2,000 $ 2,000 Net cash provided by investing activities $ 11,697 $ (7,944) $ 3,753 Decrease in cash, cash equivalents and restricted cash and cash equivalents $ (59,208) $ 1,050 $ (58,158) Cash, cash equivalents and restricted cash and cash equivalents at beginning of period $ 163,266 $ (32,818) $ 130,448 Cash, cash equivalents and restricted cash and cash equivalents at end of period $ 104,058 $ (31,768) $ 72,290 Reconciliation of cash, cash equivalents and restricted cash and cash equivalents Restricted cash and cash equivalents included in restricted investments and other non-current assets $ 33,026 $ (31,768) $ 1,258 Total cash, cash equivalents and restricted cash and cash equivalents $ 104,058 $ (31,768) $ 72,290 For the year ended December 31, 2021 in thousands As previously reported Adjustment As revised Cash flows from operating activities: Changes in operating assets and liabilities: Prepaid expenses and other assets $ 1,578 $ 179 $ 1,757 Net cash used in operating activities $ (207,034) $ 179 $ (206,855) Cash flows from investing activities: Purchases of restricted investments $ — $ (32,997) $ (32,997) Net cash used in (provided by) investing activities $ 15,411 $ (32,997) $ (17,586) Decrease in cash, cash equivalents and restricted cash and cash equivalents $ 163,266 $ (32,818) $ 130,448 Cash, cash equivalents and restricted cash and cash equivalents at end of period $ 163,266 $ (32,818) $ 130,448 Reconciliation of cash, cash equivalents and restricted cash and cash equivalents Restricted cash and cash equivalents included in restricted investments and other non-current assets $ 32,852 $ (32,818) $ 34 Total cash, cash equivalents and restricted cash and cash equivalents $ 163,266 $ (32,818) $ 130,448
Estimated Useful Lives of Assets	Depreciation and amortization is calculated using the straight-line method over the estimated useful lives of the assets, which are as follows: Asset Estimated useful life Building 40 years Computer equipment and software 3 years Furniture and fixtures 2-5 years Laboratory equipment 5-10 years Leasehold improvements Shorter of the useful life or remaining lease term
Changes in Balances of Company's Receivables and Contract Liabilities	For the years ending 2023, 2022 and 2021, collaborative arrangement revenue consisted of the following: Year Ended December 31, 2023 2022 2021 U.S. Abecma collaboration with BMS $ 50,010 $ 12,781 $ 19,425 Collaboration with Regeneron 21,591 19,577 7,496 Total collaborative arrangement revenue $ 71,601 $ 32,358 $ 26,921 The following table presents changes in the balances of the Company’s BMS receivables and contract liabilities during the twelve months ended December 31, 2023 (in thousands): Balance at December 31, Additions Deductions Balance at December 31, Receivables $ 4,537 $ 46,212 $ (50,749) $ — Contract liabilities: Deferred revenue $ — $ — $ — $ — The following table summarizes the allocation of the transaction price to each performance obligation and the amount of the allocated transaction price that is unsatisfied or partially unsatisfied as of December 31, 2023, which the Company expects to recognize as revenue as the targets progress through each of the target’s respective IND filing (in thousands): Performance Obligation Allocation of Transaction Price Unsatisfied Portion of Transaction Price MUC16 Mono/Combo & Next Gen Therapies $ 1,905 $ 144 MAGE-A4 178 10 Early Research Target (1) 8,701 7,743 Early Research Target (2) 475 424 Total $ 11,259 $ 8,321
Summary of Financial Statement Presentation for Payments To and From Collaborators Not Deemed as Customers	For the years ending 2023, 2022 and 2021, service revenue consisted of the following: Year Ended December 31, 2023 2022 2021 ide-cel ex-U.S. service revenue from BMS $ 14,751 $ 13,226 $ 16,895 bb21717 license and manufacturing service revenue from BMS — 35,762 — Service revenue from December 2021 agreement with Novo Nordisk 5,765 6,501 — Other 3,628 — 4,486 Total service revenue $ 24,144 $ 55,489 $ 21,381 In arrangements where the Company does not deem its collaborator to be its customer, payments to and from its collaborator are presented in the consolidated and combined statements of operations and comprehensive loss based on the nature of the payments, as summarized in the table and further described below. Nature of Payment Statement of Operations Presentation The Company's share of net profits in connection with commercialization of products Collaborative arrangement revenue The Company's share of net losses in connection with commercialization of products Share of collaboration loss Net reimbursement to the Company for research and development expenses Collaborative arrangement revenue Net reimbursement to the collaborator for research and development expenses Research and development expense The following table summarizes the net collaboration revenue recognized or expense incurred for the joint ide-cel development efforts in the U.S. under ASC 808, including revenue or expense related to the combined performance obligation for license and manufacturing of ide-cel in the U.S. for the years ended December 31, 2023, 2022 and 2021 (in thousands): For the years ended December 31, 2023 2022 2021 ASC 808 ide-cel license and manufacturing revenue - U.S. (1)(2) $ — $ — $ 533 ASC 808 ide-cel license and manufacturing expense - U.S. (1) $ — $ — $ — (1) As noted above, the calculation of collaborative arrangement activity to be recognized for joint ide-cel efforts in the United States is performed on a quarterly basis. The calculation is independent of previous activity, which may result in fluctuations between revenue and expense recognition period over period, depending on the varying extent of effort performed by each party during the period. The following tables summarize the components utilized in the Company’s quarterly calculation of collaborative arrangement revenue or share of collaboration loss under the BMS collaboration arrangement for the years ended December 31, 2023, 2022 and 2021 (in thousands). The amounts reported for these periods represent the Company’s share of BMS’ Abecma product revenue, cost of goods sold, and selling costs, along with reimbursement by BMS of commercial costs incurred by the Company, and exclude expenses related to ongoing development, which are separately reflected in the consolidated and combined statements of operations and comprehensive loss as described below. Three months ended Year ended Abecma U.S. Collaboration Profit/Loss Share March 31, 2023 June 30, 2023 September 30, 2023 December 31, 2023 December 31, 2023 2seventy's share of profits (losses), net of 2seventy's share of BMS costs for commercial activities $ 21,581 $ 23,272 $ (582) $ 1,366 $ 45,637 Reimbursement from BMS for 2seventy costs of commercial manufacturing and commercial activities 1,380 1,271 1,118 604 4,373 Collaborative arrangement revenue (1) $ 22,961 $ 24,543 $ 536 $ 1,970 $ 50,010 Share of collaboration loss (1) $ — $ — $ — $ — $ — Three months ended Year ended Abecma U.S. Collaboration Profit/Loss Share March 31, 2022 June 30, 2022 September 30, 2022 December 31, 2022 December 31, 2022 2seventy's share of profits (losses), net of 2seventy's share of BMS costs for commercial activities $ (6,709) $ (5,931) $ 2,849 $ 7,286 $ (2,505) Reimbursement from BMS for 2seventy costs of commercial manufacturing and commercial activities 1,357 1,641 1,215 1,431 5,644 Collaborative arrangement revenue (1) $ — $ — $ 4,064 $ 8,717 $ 12,781 Share of collaboration loss (1) $ (5,352) $ (4,290) $ — $ — $ (9,642) Three months ended Year ended Abecma U.S. Collaboration Profit/Loss Share March 31, 2021 (2) June 30, 2021 September 30, 2021 December 31, 2021 December 31, 2021 2seventy's share of profits (losses), net of 2seventy's share of BMS costs for commercial activities $ — $ (11,766) $ 9,762 $ 7,901 $ 5,897 Reimbursement from BMS for 2seventy costs of commercial manufacturing and commercial activities — 1,695 845 917 3,457 Collaborative arrangement revenue (1) $ — $ — $ 10,607 $ 8,818 $ 19,425 Share of collaboration loss (1) $ — $ (10,071) $ — $ — $ (10,071) (1) As noted above, the calculation is performed on a quarterly basis and consists of 2seventy's share of profits, net of 2seventy's share of BMS costs for commercial activities, offset by reimbursement from BMS for 2seventy commercial activities. The calculation is independent of previous activity, which may result in fluctuations between revenue and expense recognition period over period. (2) In March 2021, Abecma (idecabtagene vicleucel; ide-cel) was approved by the FDA in the United States for the treatment of adults with multiple myeloma who have received at least four prior lines of therapy, including an immunomodulatory agent, a proteasome inhibitor, and an anti-CD38 monoclonal antibody. The first sale of Abecma occurred in the second quarter of 2021. The following table summarizes the amounts associated with the research activities under the collaboration included in research and development expense or recognized as collaborative arrangement revenue for each quarter during the years ended December 31, 2023, 2022, and 2021 (in thousands): Three months ended Year ended Abecma U.S. Collaboration Net R&D Expenses March 31, 2023 June 30, 2023 September 30. 2023 December 31, 2023 December 31, 2023 2seventy's obligation for its share of BMS research and development expenses $ (9,461) $ (7,195) $ (6,980) $ (7,237) $ (30,873) Reimbursement from BMS for 2seventy research and development expenses 4,590 1,543 860 151 7,144 Net R&D expense (1) $ (4,871) $ (5,652) $ (6,120) $ (7,086) $ (23,729) Three months ended Year ended Abecma U.S. Collaboration Net R&D Expenses March 31, 2022 June 30, 2022 September 30. 2022 December 31, 2022 December 31, 2022 2seventy's obligation for its share of BMS research and development expenses $ (8,118) $ (7,418) $ (10,672) $ (8,197) $ (34,405) Reimbursement from BMS for 2seventy research and development expenses 1,225 1,955 1,420 1,960 6,560 Net R&D expense (1) $ (6,893) $ (5,463) $ (9,252) $ (6,237) $ (27,845) Three months ended Year ended Abecma U.S. Collaboration Net R&D Expenses March 31, 2021 June 30, 2021 September 30. 2021 December 31, 2021 December 31, 2021 2seventy's obligation for its share of BMS research and development expenses $ (22,110) $ (11,559) $ (9,398) $ (8,780) $ (51,847) Reimbursement from BMS for 2seventy research and development expenses 4,752 2,366 3,738 945 11,801 Net R&D expense (1) $ (17,358) $ (9,193) $ (5,660) $ (7,835) $ (40,046) (1) As noted above, the calculation is performed on a quarterly basis and consists of 2seventy's obligation for its share of BMS research and development expenses, offset by reimbursement from BMS for 2seventy research and development expenses. The Company accounts for any ex-U.S. activities under the Amended Ide-cel CCPS pursuant to ASC 606. The following table summarizes the revenue recognized related to ide-cel ex-U.S. activities for the years ended December 31, 2023, 2022, and 2021 (in thousands). These amounts are reflected in service revenue in the consolidated and combined statements of operations and comprehensive loss: For the years ended December 31, 2023 2022 2021 ASC 606 ide-cel license and manufacturing revenue – ex-U.S. (included as a component of service revenue) (1) $ 14,751 $ 13,226 $ 16,895 (1) These amounts include reimbursements from BMS to the Company for the Company’s ex-U.S. quality and other manufacturing costs associated with the manufacture of Abecma inventory.

Marketable securities (Tables)

Marketable securities (Tables)	12 Months Ended
Marketable securities (Tables)	Dec. 31, 2023
Investments, Debt and Equity Securities [Abstract]
Summary of Marketable Securities	The following table summarizes the marketable securities held at December 31, 2023 and December 31, 2022 (in thousands): Amortized Unrealized gains Unrealized losses Fair Value December 31, 2023 U.S. government agency securities and $ 101,566 $ 144 $ (85) $ 101,625 Commercial paper 45,188 34 — 45,222 Total $ 146,754 $ 178 $ (85) $ 146,847 December 31, 2022 U.S. government agency securities and $ 120,739 $ 3 $ (1,963) $ 118,779 Corporate bonds 2,524 — (26) 2,498 Commercial paper 75,491 3 (119) 75,375 Total $ 198,754 $ 6 $ (2,108) $ 196,652
Summary of Available-for-sale Debt Securities in a Continuous Unrealized Loss Position	The following table summarizes available-for-sale debt securities in a continuous unrealized loss position for less than and greater than twelve months, and for which an allowance for credit losses has not been recorded at December 31, 2023 and December 31, 2022 (in thousands): Less than 12 months 12 months or greater Total Description Fair value Unrealized losses Fair value Unrealized losses Fair value Unrealized losses December 31, 2023 U.S. government agency securities and $ 45,850 $ (60) $ 1,475 $ (25) $ 47,325 $ (85) Total $ 45,850 $ (60) $ 1,475 $ (25) $ 47,325 $ (85) December 31, 2022 U.S. government agency securities and $ 28,749 $ (159) $ 86,176 $ (1,804) $ 114,925 $ (1,963) Corporate bonds — — 2,498 (26) 2,498 (26) Commercial paper 62,636 (119) — — 62,636 (119) Total $ 91,385 $ (278) $ 88,674 $ (1,830) $ 180,059 $ (2,108)
Restricted Investments	The following table summarizes restricted investments held at December 31, 2023 and December 31, 2022 (in thousands): Amortized Unrealized gains Unrealized losses Fair Value December 31, 2023 U.S. government agency securities and $ 33,072 $ 67 $ (365) $ 32,774 Total $ 33,072 $ 67 $ (365) $ 32,774 December 31, 2022 U.S. government agency securities and $ 32,880 $ — $ (1,112) $ 31,768 Total $ 32,880 $ — $ (1,112) $ 31,768
Restricted Investments, Available-for-Sale, Unrealized Loss Position, Fair Value	The following table summarizes restricted investments in a continuous unrealized loss position for less than and greater than twelve months, and for which an allowance for credit losses has not been recorded at December 31, 2023 and December 31, 2022 (in thousands): Less than 12 months 12 months or greater Total Description Fair value Unrealized losses Fair value Unrealized losses Fair value Unrealized losses December 31, 2023 U.S. government agency securities and $ 3,496 $ (4) $ 13,266 $ (361) $ 16,762 $ (365) Total $ 3,496 $ (4) $ 13,266 $ (361) $ 16,762 $ (365) December 31, 2022 U.S. government agency securities and $ 1,942 $ (27) $ 29,826 $ (1,085) $ 31,768 $ (1,112) Total $ 1,942 $ (27) $ 29,826 $ (1,085) $ 31,768 $ (1,112)

Fair value measurements (Tables

Fair value measurements (Tables)	12 Months Ended
Fair value measurements (Tables)	Dec. 31, 2023
Fair Value Disclosures [Abstract]
Summary of Assets and Liabilities Measured at Fair Value on a Recurring Basis	The following table sets forth the Company’s assets and liabilities that are measured at fair value on a recurring basis as of December 31, 2023 and 2022 (in thousands): Total Quoted prices in active markets (Level 1) Significant other observable inputs (Level 2) Significant unobservable inputs (Level 3) December 31, 2023 Assets: Cash and cash equivalents $ 74,958 $ 74,958 $ — $ — Marketable securities: U.S. government agency securities and treasuries 101,625 — 101,625 — Commercial paper 45,222 — 45,222 — Restricted cash and cash equivalents 1,725 1,725 — Restricted investments 32,774 — 32,774 — Total assets $ 256,304 $ 76,683 $ 179,621 $ — Liabilities: Contingent consideration $ 2,415 $ — $ — $ 2,415 Total liabilities $ 2,415 $ — $ — $ 2,415 December 31, 2022 Assets: Cash and cash equivalents $ 71,032 $ 71,032 $ — $ — Marketable securities: U.S. government agency securities and treasuries 118,779 — 118,779 — Corporate bonds 2,498 — 2,498 — Commercial paper 75,375 — 75,375 — Restricted cash and cash equivalents 1,258 1,258 — — Restricted investments 31,768 — 31,768 — Total assets $ 300,710 $ 72,290 $ 228,420 $ — Liabilities: Contingent consideration $ 2,180 $ — $ — $ 2,180 Total liabilities $ 2,180 $ — $ — $ 2,180
Summary of Available-for-sale Debt Securities in a Continuous Unrealized Loss Position	The following table summarizes available-for-sale debt securities in a continuous unrealized loss position for less than and greater than twelve months, and for which an allowance for credit losses has not been recorded at December 31, 2023 and December 31, 2022 (in thousands): Less than 12 months 12 months or greater Total Description Fair value Unrealized losses Fair value Unrealized losses Fair value Unrealized losses December 31, 2023 U.S. government agency securities and $ 45,850 $ (60) $ 1,475 $ (25) $ 47,325 $ (85) Total $ 45,850 $ (60) $ 1,475 $ (25) $ 47,325 $ (85) December 31, 2022 U.S. government agency securities and $ 28,749 $ (159) $ 86,176 $ (1,804) $ 114,925 $ (1,963) Corporate bonds — — 2,498 (26) 2,498 (26) Commercial paper 62,636 (119) — — 62,636 (119) Total $ 91,385 $ (278) $ 88,674 $ (1,830) $ 180,059 $ (2,108)
Roll-Forward of Fair Value of the Company's Contingent Consideration Obligations	The table below provides a roll-forward of fair value of the Company’s contingent consideration obligations that include Level 3 inputs (in thousands): Year ended December 31, 2023 2022 Beginning balance $ 2,180 $ 1,948 Additions — — Changes in fair value 235 232 Payments — — Ending balance $ 2,415 $ 2,180

Property, plant and equipment_2

Property, plant and equipment, net (Tables)	12 Months Ended
Property, plant and equipment, net (Tables)	Dec. 31, 2023
Property, Plant and Equipment [Abstract]
Summary of Property, Plant and Equipment, Net	Property, plant and equipment, net, consists of the following (in thousands): As of December 31, 2023 2022 Computer equipment and software 6,156 5,670 Office equipment 6,726 6,159 Laboratory equipment 43,209 36,216 Leasehold improvements 58,832 27,416 Construction-in-progress 138 28,112 Total property, plant and equipment 115,061 103,573 Less accumulated depreciation and amortization (56,911) (47,838) Property, plant and equipment, net $ 58,150 $ 55,735

Accrued expenses and other cu_2

Accrued expenses and other current liabilities (Tables)	12 Months Ended
Accrued expenses and other current liabilities (Tables)	Dec. 31, 2023
Payables and Accruals [Abstract]
Summary of Accrued Expenses and Other Current Liabilities	Accrued expenses and other current liabilities consist of the following (in thousands): As of December 31, 2023 2022 Royalties $ 9,702 $ 13,094 Collaboration research costs 5,681 2,005 Employee compensation 4,639 14,845 Manufacturing costs 1,764 17,962 Clinical and contract research organization costs 990 1,619 Property, plant, and equipment 279 1,498 Other 2,633 3,655 Total accrued expenses and other current liabilities $ 25,688 $ 54,678

Leases (Tables)

Leases (Tables)	12 Months Ended
Leases (Tables)	Dec. 31, 2023
Leases [Abstract]
Summary of Lease Costs and Other Information Pertaining to Operating Leases	The following table contains a summary of the lease costs recognized under ASC 842 and other information pertaining to the Company’s operating leases for the years ended December 31, 2023, 2022 and 2021 (in thousands): For the year ended December 31, 2023 2022 2021 Lease cost (1) Operating lease cost $ 32,636 $ 33,738 $ 24,838 Total lease cost $ 32,636 $ 33,738 $ 24,838 Other information Operating cash flows used for operating leases $ 27,927 $ 27,140 $ 25,489 Weighted average remaining lease term 9.9 years 10.8 years 11.8 years Weighted average discount rate 5.48 % 5.47 % 5.47 % ________________ (1) Short-term lease costs incurred by the Company for the twelve months ended December 31, 2023, 2022 and 2021 were immaterial.
Summary of Future Minimum Commitments	As of December 31, 2023, future minimum commitments under ASC 842 under the Company’s operating leases were as follows (in thousands): Maturity of lease liabilities As of December 31, 2023 2024 $ 28,672 2025 29,498 2026 32,920 2027 35,086 2028 and thereafter 215,296 Total lease payments 341,472 Less: imputed interest (84,799) Total operating lease liabilities $ 256,673

Commitments and contingencies (

Commitments and contingencies (Tables)	12 Months Ended
Commitments and contingencies (Tables)	Dec. 31, 2023
Commitments and Contingencies Disclosure [Abstract]
Schedule of Non-cancelable Contractual Obligations	The following table presents non-cancelable contractual obligations arising from these arrangements: Years ended December 31, Purchase commitment 2024 $ 6,671 2025 and thereafter — Total purchase commitments $ 6,671

Stockholders' equity (Tables)

Stockholders' equity (Tables)	12 Months Ended
Stockholders' equity (Tables)	Dec. 31, 2023
Equity [Abstract]
Schedule of Common Stock Reserved for Future Issuance	The Company has reserved for future issuance the following number of shares of common stock (in thousands): As of December 31, 2023 2022 Options to purchase common stock (1) 3,678 2,562 Restricted stock units (1) 2,239 1,314 2021 Stock Option and Incentive Plan 871 1,621 2021 Employee Stock Purchase Plan 552 434 Pre-funded warrants to purchase common stock 758 758 8,098 6,689 (1)

Collaborative arrangements an_2

Collaborative arrangements and strategic partnerships (Tables)	12 Months Ended
	Dec. 31, 2023
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Summary of Total Transaction Price, Allocation of Total Transaction Price to Identified Performance Obligations Under Arrangement and Amount of Transaction Price Unsatisfied	For the years ending 2023, 2022 and 2021, service revenue consisted of the following: Year Ended December 31, 2023 2022 2021 ide-cel ex-U.S. service revenue from BMS $ 14,751 $ 13,226 $ 16,895 bb21717 license and manufacturing service revenue from BMS — 35,762 — Service revenue from December 2021 agreement with Novo Nordisk 5,765 6,501 — Other 3,628 — 4,486 Total service revenue $ 24,144 $ 55,489 $ 21,381 In arrangements where the Company does not deem its collaborator to be its customer, payments to and from its collaborator are presented in the consolidated and combined statements of operations and comprehensive loss based on the nature of the payments, as summarized in the table and further described below. Nature of Payment Statement of Operations Presentation The Company's share of net profits in connection with commercialization of products Collaborative arrangement revenue The Company's share of net losses in connection with commercialization of products Share of collaboration loss Net reimbursement to the Company for research and development expenses Collaborative arrangement revenue Net reimbursement to the collaborator for research and development expenses Research and development expense The following table summarizes the net collaboration revenue recognized or expense incurred for the joint ide-cel development efforts in the U.S. under ASC 808, including revenue or expense related to the combined performance obligation for license and manufacturing of ide-cel in the U.S. for the years ended December 31, 2023, 2022 and 2021 (in thousands): For the years ended December 31, 2023 2022 2021 ASC 808 ide-cel license and manufacturing revenue - U.S. (1)(2) $ — $ — $ 533 ASC 808 ide-cel license and manufacturing expense - U.S. (1) $ — $ — $ — (1) As noted above, the calculation of collaborative arrangement activity to be recognized for joint ide-cel efforts in the United States is performed on a quarterly basis. The calculation is independent of previous activity, which may result in fluctuations between revenue and expense recognition period over period, depending on the varying extent of effort performed by each party during the period. The following tables summarize the components utilized in the Company’s quarterly calculation of collaborative arrangement revenue or share of collaboration loss under the BMS collaboration arrangement for the years ended December 31, 2023, 2022 and 2021 (in thousands). The amounts reported for these periods represent the Company’s share of BMS’ Abecma product revenue, cost of goods sold, and selling costs, along with reimbursement by BMS of commercial costs incurred by the Company, and exclude expenses related to ongoing development, which are separately reflected in the consolidated and combined statements of operations and comprehensive loss as described below. Three months ended Year ended Abecma U.S. Collaboration Profit/Loss Share March 31, 2023 June 30, 2023 September 30, 2023 December 31, 2023 December 31, 2023 2seventy's share of profits (losses), net of 2seventy's share of BMS costs for commercial activities $ 21,581 $ 23,272 $ (582) $ 1,366 $ 45,637 Reimbursement from BMS for 2seventy costs of commercial manufacturing and commercial activities 1,380 1,271 1,118 604 4,373 Collaborative arrangement revenue (1) $ 22,961 $ 24,543 $ 536 $ 1,970 $ 50,010 Share of collaboration loss (1) $ — $ — $ — $ — $ — Three months ended Year ended Abecma U.S. Collaboration Profit/Loss Share March 31, 2022 June 30, 2022 September 30, 2022 December 31, 2022 December 31, 2022 2seventy's share of profits (losses), net of 2seventy's share of BMS costs for commercial activities $ (6,709) $ (5,931) $ 2,849 $ 7,286 $ (2,505) Reimbursement from BMS for 2seventy costs of commercial manufacturing and commercial activities 1,357 1,641 1,215 1,431 5,644 Collaborative arrangement revenue (1) $ — $ — $ 4,064 $ 8,717 $ 12,781 Share of collaboration loss (1) $ (5,352) $ (4,290) $ — $ — $ (9,642) Three months ended Year ended Abecma U.S. Collaboration Profit/Loss Share March 31, 2021 (2) June 30, 2021 September 30, 2021 December 31, 2021 December 31, 2021 2seventy's share of profits (losses), net of 2seventy's share of BMS costs for commercial activities $ — $ (11,766) $ 9,762 $ 7,901 $ 5,897 Reimbursement from BMS for 2seventy costs of commercial manufacturing and commercial activities — 1,695 845 917 3,457 Collaborative arrangement revenue (1) $ — $ — $ 10,607 $ 8,818 $ 19,425 Share of collaboration loss (1) $ — $ (10,071) $ — $ — $ (10,071) (1) As noted above, the calculation is performed on a quarterly basis and consists of 2seventy's share of profits, net of 2seventy's share of BMS costs for commercial activities, offset by reimbursement from BMS for 2seventy commercial activities. The calculation is independent of previous activity, which may result in fluctuations between revenue and expense recognition period over period. (2) In March 2021, Abecma (idecabtagene vicleucel; ide-cel) was approved by the FDA in the United States for the treatment of adults with multiple myeloma who have received at least four prior lines of therapy, including an immunomodulatory agent, a proteasome inhibitor, and an anti-CD38 monoclonal antibody. The first sale of Abecma occurred in the second quarter of 2021. The following table summarizes the amounts associated with the research activities under the collaboration included in research and development expense or recognized as collaborative arrangement revenue for each quarter during the years ended December 31, 2023, 2022, and 2021 (in thousands): Three months ended Year ended Abecma U.S. Collaboration Net R&D Expenses March 31, 2023 June 30, 2023 September 30. 2023 December 31, 2023 December 31, 2023 2seventy's obligation for its share of BMS research and development expenses $ (9,461) $ (7,195) $ (6,980) $ (7,237) $ (30,873) Reimbursement from BMS for 2seventy research and development expenses 4,590 1,543 860 151 7,144 Net R&D expense (1) $ (4,871) $ (5,652) $ (6,120) $ (7,086) $ (23,729) Three months ended Year ended Abecma U.S. Collaboration Net R&D Expenses March 31, 2022 June 30, 2022 September 30. 2022 December 31, 2022 December 31, 2022 2seventy's obligation for its share of BMS research and development expenses $ (8,118) $ (7,418) $ (10,672) $ (8,197) $ (34,405) Reimbursement from BMS for 2seventy research and development expenses 1,225 1,955 1,420 1,960 6,560 Net R&D expense (1) $ (6,893) $ (5,463) $ (9,252) $ (6,237) $ (27,845) Three months ended Year ended Abecma U.S. Collaboration Net R&D Expenses March 31, 2021 June 30, 2021 September 30. 2021 December 31, 2021 December 31, 2021 2seventy's obligation for its share of BMS research and development expenses $ (22,110) $ (11,559) $ (9,398) $ (8,780) $ (51,847) Reimbursement from BMS for 2seventy research and development expenses 4,752 2,366 3,738 945 11,801 Net R&D expense (1) $ (17,358) $ (9,193) $ (5,660) $ (7,835) $ (40,046) (1) As noted above, the calculation is performed on a quarterly basis and consists of 2seventy's obligation for its share of BMS research and development expenses, offset by reimbursement from BMS for 2seventy research and development expenses. The Company accounts for any ex-U.S. activities under the Amended Ide-cel CCPS pursuant to ASC 606. The following table summarizes the revenue recognized related to ide-cel ex-U.S. activities for the years ended December 31, 2023, 2022, and 2021 (in thousands). These amounts are reflected in service revenue in the consolidated and combined statements of operations and comprehensive loss: For the years ended December 31, 2023 2022 2021 ASC 606 ide-cel license and manufacturing revenue – ex-U.S. (included as a component of service revenue) (1) $ 14,751 $ 13,226 $ 16,895 (1) These amounts include reimbursements from BMS to the Company for the Company’s ex-U.S. quality and other manufacturing costs associated with the manufacture of Abecma inventory.
Changes in Balances of Company's Receivables and Contract Liabilities	For the years ending 2023, 2022 and 2021, collaborative arrangement revenue consisted of the following: Year Ended December 31, 2023 2022 2021 U.S. Abecma collaboration with BMS $ 50,010 $ 12,781 $ 19,425 Collaboration with Regeneron 21,591 19,577 7,496 Total collaborative arrangement revenue $ 71,601 $ 32,358 $ 26,921 The following table presents changes in the balances of the Company’s BMS receivables and contract liabilities during the twelve months ended December 31, 2023 (in thousands): Balance at December 31, Additions Deductions Balance at December 31, Receivables $ 4,537 $ 46,212 $ (50,749) $ — Contract liabilities: Deferred revenue $ — $ — $ — $ — The following table summarizes the allocation of the transaction price to each performance obligation and the amount of the allocated transaction price that is unsatisfied or partially unsatisfied as of December 31, 2023, which the Company expects to recognize as revenue as the targets progress through each of the target’s respective IND filing (in thousands): Performance Obligation Allocation of Transaction Price Unsatisfied Portion of Transaction Price MUC16 Mono/Combo & Next Gen Therapies $ 1,905 $ 144 MAGE-A4 178 10 Early Research Target (1) 8,701 7,743 Early Research Target (2) 475 424 Total $ 11,259 $ 8,321

Intangible assets (Tables)

Intangible assets (Tables)	12 Months Ended
Intangible assets (Tables)	Dec. 31, 2023
Goodwill and Intangible Assets Disclosure [Abstract]
Schedule of Finite-Lived Intangible Assets	Intangible assets, net of accumulated amortization, are summarized as follows (in thousands): As of December 31, As of December 31, 2023 2022 Cost Accumulated amortization Net Cost Accumulated amortization Net Developed technology $ 30,100 $ (30,100) $ — $ 30,100 $ (30,100) $ — In-licensed rights 8,500 (1,906) 6,594 8,500 (1,198) 7,302 Total $ 38,600 $ (32,006) $ 6,594 $ 38,600 $ (31,298) $ 7,302
Schedule of Finite-Lived Intangible Assets, Future Amortization Expense	The following table summarizes the estimated future amortization for intangible assets for the next five years and thereafter (in thousands): As of December 31, 2023 2024 $ 708 2025 708 2026 708 2027 708 2028 708 2029 and thereafter 3,054 Total $ 6,594

Stock-based compensation (Table

Stock-based compensation (Tables)	12 Months Ended
Stock-based compensation (Tables)	Dec. 31, 2023
Share-Based Payment Arrangement [Abstract]
Stock-based Compensation Expense Recognized by Award Type	Stock-based compensation expense recognized by award type is as follows (in thousands): Year Ended December 31, 2023 2022 2021 Stock options $ 11,217 $ 17,784 $ 26,185 Restricted stock units 20,607 22,983 21,414 Employee stock purchase plan and other 335 273 7,030 $ 32,159 $ 41,040 $ 54,629
Schedule of Stock-Based Compensation Expense by Classification	Stock-based compensation expense by classification included within the consolidated statements of operations and comprehensive loss was as follows (in thousands): Year Ended December 31, 2023 2022 2021 Research and development $ 12,508 $ 18,333 $ 29,746 Selling, general and administrative 19,651 22,707 24,883 $ 32,159 $ 41,040 $ 54,629
Disclosure of Share-Based Compensation Arrangements by Share-Based Payment Award	The fair value of each option issued to employees was estimated at the date of grant using the Black-Scholes option pricing model with the following weighted average assumptions: Year ended December 31, Year ended December 31, 2023 2022 Expected volatility 80.9 % 78.9 % Expected term (in years) 5.9 6.1 Risk-free interest rate 4.2 % 1.8 % Expected dividend yield 0.0 % 0.0 % (a) In connection with 2seventy’s separation from bluebird bio on November 4, 2021, all option awards granted prior to 2022 were converted as part of the separation.
Summary of Stock Option Activity	The following table summarizes the stock option activity under the Company’s equity award plans for 2seventy employees: Shares Weighted-average exercise price per share Weighted-average contractual life (in years) Aggregate intrinsic value (a) (in thousands) Outstanding at December 31, 2022 2,285 $ 53.72 Granted 2,024 $ 9.52 Exercised — $ — Cancelled or forfeited (830) $ 75.11 Outstanding at December 31, 2023 3,479 $ 22.88 8.3 $ 232 Exercisable at December 31, 2023 1,113 $ 47.51 6.8 $ — Vested and expected to vest at December 31, 2023 3,479 $ 22.88 8.3 $ 232 (a) The aggregate intrinsic value is calculated as the difference between the exercise price of the underlying options and the fair value of the common stock for the options that were in the money at December 31, 2023.
Summary of Restricted Stock Unit Activity	The following table summarizes the restricted stock unit activity under the Company’s equity award plans for 2seventy employees: Shares Weighted-average grant date Unvested balance at December 31, 2022 1,270 $ 28.31 Granted 1,902 12.30 Vested (570) 34.82 Forfeited (371) 19.06 Unvested balance at December 31, 2023 2,231 $ 14.54

Related-party transactions (Tab

Related-party transactions (Tables)	12 Months Ended
Related-party transactions (Tables)	Dec. 31, 2023
Related Party Transactions [Abstract]
Schedule of Income and Expense Resulting from Imputed Charges	The income and expense recognized by the Company for periods prior to the separation resulting from these imputed charges was recorded as other income, net in the combined financial statements and was as follows: Year ended December 31, 2021 Imputed charge to bluebird bio for leases $ 14,833 Imputed charge from bluebird bio for leases (908) Imputed charge to bluebird bio for property, plant and equipment 1,891 Imputed charge from bluebird bio for property, plant and equipment (1,130) Imputed charge to bluebird bio for intangible assets 82 Other (1) $ 14,767

Income taxes (Tables)

Income taxes (Tables)	12 Months Ended
Income taxes (Tables)	Dec. 31, 2023
Income Tax Disclosure [Abstract]
Schedule Components of Loss Before Income Taxes	The components of loss before income taxes were as follows (in thousands): Year ended December 31, 2023 2022 2021 Domestic $ (217,570) $ (254,153) $ (292,213) Foreign — — — Total $ (217,570) $ (254,153) $ (292,213)
Reconciliation of Income Tax Provision (Benefit) Computed at the Statutory Federal Income Tax Rate to Effective Income Tax Rate (Provision) Benefit as Reflected in the Financial Statements	A reconciliation of income tax expense (benefit) computed at the statutory federal income tax rate to the Company’s effective income tax rate as reflected in the financial statements is as follows: Year ended December 31, 2023 2022 2021 Federal income tax expense at statutory rate 21.0 % 21.0 % 21.0 % State income tax, net of federal benefit 6.0 % 6.6 % 0.6 % Permanent differences (0.2) % (0.2) % — % Goodwill (1.2) % — % — % Stock-based compensation (9.2) % (2.5) % 0.3 % Research and development credit 5.3 % 3.2 % 0.2 % Officer compensation limitation 4.0 % (1.2) % (0.4) % Uncertain tax positions (0.4) % (0.2) % — % Other 0.7 % (0.6) % (1.1) % Change in tax rate (1.8) % 1.3 % — % Change in valuation allowance (24.2) % (27.4) % 64.8 % Separation adjustment — % — % (85.4) % Effective income tax rate (expense) benefit — % — % — %
Components of Deferred Tax Assets and Liabilities	The significant components of the Company’s deferred tax assets and liabilities are composed of the following (in thousands): Year ended December 31, 2023 2022 Deferred tax assets: U.S. net operating loss carryforwards (federal and state) $ 38,685 $ 27,251 Tax credit carryforwards (federal and state) 24,933 10,071 Capitalized license fees and research and development expenses 10,562 11,888 Deferred revenue 5,015 3,237 Stock-based compensation 11,834 16,719 Lease liabilities 67,222 73,719 Accruals and other 2,336 3,857 Capitalized research and development expenses 88,721 55,719 Total deferred tax assets 249,308 202,461 Right-of-use assets (57,087) (65,181) Fixed assets (11,499) (9,266) Less: valuation allowance (180,722) (128,014) Net deferred taxes $ — $ —
Reconciliation of Unrecognized Tax Benefits	A reconciliation of the beginning and ending amount of unrecognized tax benefits is as follows (in thousands): Unrecognized tax benefits Balance as of December 31, 2021 36 Increases (decreases) for tax positions related to current period 713 Increases (decreases) for tax positions as part of separation adjustment — Balance as of December 31, 2022 $ 749 Increases (decreases) for tax positions related to current period 1,051 Increases (decreases) for tax positions as part of separation adjustment — Balance as of December 31, 2023 $ 1,800

Net loss per share (Tables)

Net loss per share (Tables)	12 Months Ended
Net loss per share (Tables)	Dec. 31, 2023
Earnings Per Share [Abstract]
Schedule of Common Stock Equivalents Excluded from the Calculation of Diluted Net Loss per Share	The following common stock equivalents were excluded from the calculation of diluted net loss per share for the periods indicated because including them would have had an anti-dilutive effect (in thousands): Year ended December 31, 2023 2022 2021 Outstanding stock options (1) 3,678 2,562 1,490 Restricted stock units (1) 2,239 1,314 1,080 Employee Stock Purchase Plan 81 35 — 5,998 3,911 2,570 (1)

Corporate restructuring (Tables

Corporate restructuring (Tables)	12 Months Ended
Corporate restructuring (Tables)	Dec. 31, 2023
Restructuring and Related Activities [Abstract]
Summary of Accrued Liability Activity	The following table summarizes the accrued liabilities activity recorded in connection with the Restructuring Plan as of December 31, 2023: As of December 31, 2023 Beginning balance $ — Total estimated expenses 8,614 Expenses paid from inception through December 31, 2023 (6,253) Reversal of excess accrual through December 31, 2023 — Remaining accrual at December 31, 2023 (1) $ 2,361 (1)

Goodwill impairment (Tables)

Goodwill impairment (Tables)	12 Months Ended
Goodwill impairment (Tables)	Dec. 31, 2023
Goodwill and Intangible Assets Disclosure [Abstract]
Schedule of Goodwill	The following table summarizes the activity of goodwill for the twelve months ended December 31, 2023 (in thousands): Balance at December 31, Additions Deductions Balance at December 31, Goodwill $ 12,056 $ — $ (12,056) $ —

Description of the business - N

Description of the business - Narrative (Detail) $ in Thousands	1 Months Ended	12 Months Ended
	Nov. 30, 2022 USD ($)	Dec. 31, 2023 USD ($) employee	Dec. 31, 2022 USD ($)	Dec. 31, 2021 USD ($)	Sep. 30, 2021
Consolidation, Less than Wholly Owned Subsidiary, Parent Ownership Interest, Effects of Changes, Net [Line Items]
Conversion ratio		0.3333			0.3333
Net loss		$ 217,570	$ 254,153	$ 292,213
Net cash used in operating activities		166,858	$ 228,140	$ 206,855
Cash, cash equivalents, and marketable securities		$ 221,800
ATM Facility \| Cowen and Company, LLC
Consolidation, Less than Wholly Owned Subsidiary, Parent Ownership Interest, Effects of Changes, Net [Line Items]
Aggregate gross proceeds (up to)	$ 150,000
2seventy bio Securities Corporation
Consolidation, Less than Wholly Owned Subsidiary, Parent Ownership Interest, Effects of Changes, Net [Line Items]
Number of employees \| employee		0

Summary of significant accoun_4

Summary of significant accounting policies and basis of presentation - Narrative (Detail) $ in Thousands	1 Months Ended	12 Months Ended
	Sep. 30, 2023	Dec. 31, 2023 USD ($) segment shares	Dec. 31, 2022 USD ($) shares	Dec. 31, 2021 USD ($)	Nov. 04, 2021 USD ($)	Nov. 03, 2021 shares
Restricted Cash and Cash Equivalents Items [Line Items]
Transfers from bluebird bio		$ 0	$ 0	$ 354,889
Net assets					$ 384,500
Number of operating segments \| segment		1
Accumulated goodwill impairment loss		$ 12,100
Interest income		$ 12,413	$ 2,932	88
Common stock, shares outstanding (in shares) \| shares		50,632,000	37,928,000			0
Net cash provided by (used in) investing activities		$ 43,861	$ 3,753	(17,586)
Net cash used in operating activities		(166,858)	(228,140)	(206,855)
Restricted cash and cash equivalents included in restricted investments and other non-current assets		1,725	1,258	34
Adjustment
Restricted Cash and Cash Equivalents Items [Line Items]
Net cash provided by (used in) investing activities		8,000	(7,944)	(32,997)
Net cash used in operating activities		8,000	8,994	179
Restricted cash and cash equivalents included in restricted investments and other non-current assets			(31,768)	$ (32,818)
August 2023 Reduction
Restricted Cash and Cash Equivalents Items [Line Items]
Workforce reduction percentage	40%
Letter of credit
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash and cash equivalents included in restricted investments and other non-current assets		$ 32,800	$ 30,000

Summary of significant accoun_5

Summary of significant accounting policies and basis of presentation - Correction of Immaterial Error (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Changes in operating assets and liabilities:
Prepaid expenses and other assets	$ 15,735	$ (7,332)	$ 1,757
Accrued expenses and other liabilities	(27,713)	(140)	15,733
Net cash used in operating activities	(166,858)	(228,140)	(206,855)
Cash flows from investing activities:
Purchases of property, plant and equipment	(13,865)	(30,810)	(11,575)
Purchase of restricted investments	(18,430)	(1,976)	(32,997)
Proceeds from maturities of restricted investments	18,000	2,000	0
Net cash provided by (used in) investing activities	43,861	3,753	(17,586)
Increase (decrease) in cash, cash equivalents and restricted cash and cash equivalents	4,393	(58,158)	130,448
Cash, cash equivalents and restricted cash and cash equivalents at beginning of year	72,290	130,448	0
Cash, cash equivalents and restricted cash and cash equivalents at end of year	76,683	72,290	130,448
Reconciliation of cash, cash equivalents and restricted cash and cash equivalents:
Restricted cash and cash equivalents included in restricted investments and other non-current assets	1,725	1,258	34
Total cash, cash equivalents and restricted cash and cash equivalents	76,683	72,290	130,448
As previously reported
Changes in operating assets and liabilities:
Prepaid expenses and other assets		(8,356)	1,578
Accrued expenses and other liabilities		(8,110)
Net cash used in operating activities		(237,134)	(207,034)
Cash flows from investing activities:
Purchases of property, plant and equipment		(22,842)
Purchase of restricted investments		0	0
Proceeds from maturities of restricted investments		0
Net cash provided by (used in) investing activities		11,697	15,411
Increase (decrease) in cash, cash equivalents and restricted cash and cash equivalents		(59,208)	163,266
Cash, cash equivalents and restricted cash and cash equivalents at beginning of year	104,058	163,266
Cash, cash equivalents and restricted cash and cash equivalents at end of year		104,058	163,266
Reconciliation of cash, cash equivalents and restricted cash and cash equivalents:
Restricted cash and cash equivalents included in restricted investments and other non-current assets		33,026	32,852
Total cash, cash equivalents and restricted cash and cash equivalents		104,058	163,266
Adjustment
Changes in operating assets and liabilities:
Prepaid expenses and other assets		1,024	179
Accrued expenses and other liabilities		7,970
Net cash used in operating activities	8,000	8,994	179
Cash flows from investing activities:
Purchases of property, plant and equipment		(7,968)
Purchase of restricted investments		(1,976)	(32,997)
Proceeds from maturities of restricted investments		2,000
Net cash provided by (used in) investing activities	8,000	(7,944)	(32,997)
Increase (decrease) in cash, cash equivalents and restricted cash and cash equivalents		1,050	(32,818)
Cash, cash equivalents and restricted cash and cash equivalents at beginning of year	$ (31,768)	(32,818)
Cash, cash equivalents and restricted cash and cash equivalents at end of year		(31,768)	(32,818)
Reconciliation of cash, cash equivalents and restricted cash and cash equivalents:
Restricted cash and cash equivalents included in restricted investments and other non-current assets		(31,768)	(32,818)
Total cash, cash equivalents and restricted cash and cash equivalents		$ (31,768)	$ (32,818)

Summary of significant accoun_6

Summary of significant accounting policies and basis of presentation - Estimated Useful Lives of Assets (Detail)	Dec. 31, 2023
Building
Property, Plant and Equipment [Line Items]
Estimated useful life	40 years
Computer equipment and software
Property, Plant and Equipment [Line Items]
Estimated useful life	3 years
Furniture and fixtures \| Minimum
Property, Plant and Equipment [Line Items]
Estimated useful life	2 years
Furniture and fixtures \| Maximum
Property, Plant and Equipment [Line Items]
Estimated useful life	5 years
Laboratory equipment \| Minimum
Property, Plant and Equipment [Line Items]
Estimated useful life	5 years
Laboratory equipment \| Maximum
Property, Plant and Equipment [Line Items]
Estimated useful life	10 years

Summary of significant accoun_7

Summary of significant accounting policies and basis of presentation - Service Revenue (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Service revenue
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Service revenue	$ 24,144	$ 55,489	$ 21,381
Ide-cel license and manufacturing services \| Bristol-Myers Squibb \| Outside of U.S. \| License and manufacturing services
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Service revenue	14,751	13,226	16,895
bb21217 license and manufacturing services \| Bristol-Myers Squibb \| License and manufacturing services
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Service revenue	0	35,762	0
Novo Collaboration and License Agreement \| Novo Nordisk A/S
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Service revenue	5,765	6,501	0
Other
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Service revenue	$ 3,628	$ 0	$ 4,486

Summary of significant accoun_8

Summary of significant accounting policies and basis of presentation - Collaborative Arrangement Revenue (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Collaborative arrangement revenue	$ 71,601	$ 32,358	$ 26,921
Bristol-Myers Squibb \| ABECMA Commercial Activities
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Collaborative arrangement revenue	50,010	12,781	19,425
Regeneron Collaboration Agreement \| Collaboration Arrangement
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Collaborative arrangement revenue	$ 21,591	$ 19,577	$ 7,496

Marketable securities - Summary

Marketable securities - Summary of Marketable Securities (Details) - USD ($) $ in Thousands	Dec. 31, 2023	Dec. 31, 2022
Marketable Securities [Line Items]
Amortized cost/ cost	$ 146,754	$ 198,754
Unrealized gains	178	6
Unrealized losses	(85)	(2,108)
Fair Value	146,847	196,652
U.S. government agency securities and treasuries
Marketable Securities [Line Items]
Amortized cost/ cost	101,566	120,739
Unrealized gains	144	3
Unrealized losses	(85)	(1,963)
Fair Value	101,625	118,779
Corporate bonds
Marketable Securities [Line Items]
Amortized cost/ cost		2,524
Unrealized gains		0
Unrealized losses		(26)
Fair Value		2,498
Commercial paper
Marketable Securities [Line Items]
Amortized cost/ cost	45,188	75,491
Unrealized gains	34	3
Unrealized losses	0	(119)
Fair Value	$ 45,222	$ 75,375

Marketable securities - Schedul

Marketable securities - Schedule of Unrealized Loss on Investments (Details) - USD ($) $ in Thousands	Dec. 31, 2023	Dec. 31, 2022
Fair value
Less than 12 months	$ 45,850	$ 91,385
12 months or greater	1,475	88,674
Total	47,325	180,059
Unrealized losses
Less than 12 months	(60)	(278)
12 months or greater	(25)	(1,830)
Total	(85)	(2,108)
U.S. government agency securities and treasuries
Fair value
Less than 12 months	45,850	28,749
12 months or greater	1,475	86,176
Total	47,325	114,925
Unrealized losses
Less than 12 months	(60)	(159)
12 months or greater	(25)	(1,804)
Total	$ (85)	(1,963)
Corporate bonds
Fair value
Less than 12 months		0
12 months or greater		2,498
Total		2,498
Unrealized losses
Less than 12 months		0
12 months or greater		(26)
Total		(26)
Commercial paper
Fair value
Less than 12 months		62,636
12 months or greater		0
Total		62,636
Unrealized losses
Less than 12 months		(119)
12 months or greater		0
Total		$ (119)

Marketable securities - Summa_2

Marketable securities - Summary of Restricted Investments (Details) - USD ($) $ in Thousands	Dec. 31, 2023	Dec. 31, 2022
Debt Securities, Available-for-sale [Line Items]
Amortized cost/ cost	$ 33,072	$ 32,880
Unrealized gains	67	0
Unrealized losses	(365)	(1,112)
Fair Value	32,774	31,768
U.S. government agency securities and treasuries
Debt Securities, Available-for-sale [Line Items]
Amortized cost/ cost	33,072	32,880
Unrealized gains	67	0
Unrealized losses	(365)	(1,112)
Fair Value	$ 32,774	$ 31,768

Marketable securities - Sched_2

Marketable securities - Schedule of Unrealized Loss on Restricted Investments (Details) - USD ($) $ in Thousands	Dec. 31, 2023	Dec. 31, 2022
Fair value
Less than 12 months	$ 3,496	$ 1,942
12 months or greater	13,266	29,826
Total	16,762	31,768
Unrealized losses
Less than 12 months	(4)	(27)
12 months or greater	(361)	(1,085)
Total	(365)	(1,112)
U.S. government agency securities and treasuries
Fair value
Less than 12 months	3,496	1,942
12 months or greater	13,266	29,826
Total	16,762	31,768
Unrealized losses
Less than 12 months	(4)	(27)
12 months or greater	(361)	(1,085)
Total	$ (365)	$ (1,112)

Marketable securities - Narrati

Marketable securities - Narrative (Details) - USD ($)	12 Months Ended
Marketable securities - Narrative (Details) - USD ($)	Dec. 31, 2023	Dec. 31, 2022
Investments, Debt and Equity Securities [Abstract]
Interest receivable	$ 800,000	$ 300,000
Interest receivable, write-off	0	0
Realized gain (loss) recognized on sale	$ 0	$ 0

Fair value measurements - Recor

Fair value measurements - Recorded Amount of Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) - USD ($) $ in Thousands	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Assets:
Restricted cash and cash equivalents	$ 76,683	$ 72,290	$ 130,448	$ 0
Fair value, measurements, recurring
Assets:
Cash and cash equivalents	74,958	71,032
Restricted cash and cash equivalents	1,725	1,258
Restricted investments	32,774	31,768
Total assets	256,304	300,710
Liabilities:
Contingent consideration	2,415	2,180
Total liabilities	2,415	2,180
Fair value, measurements, recurring \| U.S. government agency securities and treasuries
Assets:
Marketable securities:	101,625	118,779
Fair value, measurements, recurring \| Corporate bonds
Assets:
Marketable securities:		2,498
Fair value, measurements, recurring \| Commercial paper
Assets:
Marketable securities:	45,222	75,375
Fair value, measurements, recurring \| Quoted prices in active markets (Level 1)
Assets:
Cash and cash equivalents	74,958	71,032
Restricted cash and cash equivalents	1,725	1,258
Restricted investments	0	0
Total assets	76,683	72,290
Liabilities:
Contingent consideration	0	0
Total liabilities	0	0
Fair value, measurements, recurring \| Quoted prices in active markets (Level 1) \| U.S. government agency securities and treasuries
Assets:
Marketable securities:	0	0
Fair value, measurements, recurring \| Quoted prices in active markets (Level 1) \| Corporate bonds
Assets:
Marketable securities:		0
Fair value, measurements, recurring \| Quoted prices in active markets (Level 1) \| Commercial paper
Assets:
Marketable securities:	0	0
Fair value, measurements, recurring \| Significant other observable inputs (Level 2)
Assets:
Cash and cash equivalents	0	0
Restricted cash and cash equivalents	0	0
Restricted investments	32,774	31,768
Total assets	179,621	228,420
Liabilities:
Contingent consideration	0	0
Total liabilities	0	0
Fair value, measurements, recurring \| Significant other observable inputs (Level 2) \| U.S. government agency securities and treasuries
Assets:
Marketable securities:	101,625	118,779
Fair value, measurements, recurring \| Significant other observable inputs (Level 2) \| Corporate bonds
Assets:
Marketable securities:		2,498
Fair value, measurements, recurring \| Significant other observable inputs (Level 2) \| Commercial paper
Assets:
Marketable securities:	45,222	75,375
Fair value, measurements, recurring \| Significant unobservable inputs (Level 3)
Assets:
Cash and cash equivalents	0	0
Restricted cash and cash equivalents		0
Restricted investments	0	0
Total assets	0	0
Liabilities:
Contingent consideration	2,415	2,180
Total liabilities	2,415	2,180
Fair value, measurements, recurring \| Significant unobservable inputs (Level 3) \| U.S. government agency securities and treasuries
Assets:
Marketable securities:	0	0
Fair value, measurements, recurring \| Significant unobservable inputs (Level 3) \| Corporate bonds
Assets:
Marketable securities:		0
Fair value, measurements, recurring \| Significant unobservable inputs (Level 3) \| Commercial paper
Assets:
Marketable securities:	$ 0	$ 0

Fair value measurements - Roll-

Fair value measurements - Roll-Forward of Fair Value of the Company's Contingent Consideration Obligations (Detail) - Significant unobservable inputs (Level 3) - Contingent consideration obligations - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	$ 2,180	$ 1,948
Additions	0	0
Changes in fair value	235	232
Payments	0	0
Ending balance	$ 2,415	$ 2,180

Property, plant and equipment_3

Property, plant and equipment, net - Summary of Property, Plant and Equipment, Net (Detail) - USD ($) $ in Thousands	Dec. 31, 2023	Dec. 31, 2022
Property, Plant and Equipment [Line Items]
Total property, plant and equipment	$ 115,061	$ 103,573
Less accumulated depreciation and amortization	(56,911)	(47,838)
Property, plant and equipment, net	58,150	55,735
Computer equipment and software
Property, Plant and Equipment [Line Items]
Total property, plant and equipment	6,156	5,670
Office equipment
Property, Plant and Equipment [Line Items]
Total property, plant and equipment	6,726	6,159
Laboratory equipment
Property, Plant and Equipment [Line Items]
Total property, plant and equipment	43,209	36,216
Leasehold improvements
Property, Plant and Equipment [Line Items]
Total property, plant and equipment	58,832	27,416
Construction-in-progress
Property, Plant and Equipment [Line Items]
Total property, plant and equipment	$ 138	$ 28,112

Property, plant and equipment_4

Property, plant and equipment, net - Narrative (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Property, Plant and Equipment [Line Items]
Depreciation	$ 9,600	$ 8,900	$ 12,100
Construction in progress	115,061	103,573
Construction-in-progress
Property, Plant and Equipment [Line Items]
Construction in progress	138	28,112
Construction-in-progress \| Cambridge, Massachusetts
Property, Plant and Equipment [Line Items]
Construction in progress	$ 36,600	$ 27,000

Accrued expenses and other cu_3

Accrued expenses and other current liabilities - Summary of Accrued Expenses and Other Current Liabilities (Detail) - USD ($) $ in Thousands	Dec. 31, 2023	Dec. 31, 2022
Payables and Accruals [Abstract]
Royalties	$ 9,702	$ 13,094
Collaboration research costs	5,681	2,005
Employee compensation	4,639	14,845
Manufacturing costs	1,764	17,962
Clinical and contract research organization costs	990	1,619
Property, plant, and equipment	279	1,498
Other	2,633	3,655
Total accrued expenses and other current liabilities	$ 25,688	$ 54,678

Accrued expenses and other cu_4

Accrued expenses and other current liabilities - Narrative (Details) $ in Millions	Dec. 31, 2022 USD ($)
Resilience
Commitments And Contingencies Disclosure [Line Items]
Accrued liabilities	$ 14.8

Leases - Narrative (Details)

Leases - Narrative (Details) $ in Thousands			1 Months Ended				12 Months Ended
Leases - Narrative (Details) $ in Thousands	Apr. 01, 2026 USD ($) $ / ft²	Oct. 01, 2016 USD ($) ft² $ / ft²	Nov. 30, 2021 USD ($) $ / ft²	Sep. 30, 2020 ft² $ / ft²	Sep. 30, 2019 ft² lease_term $ / ft²	Jul. 31, 2018 ft² $ / ft²	Dec. 31, 2023 USD ($)	Dec. 31, 2022 USD ($)	Dec. 31, 2021 USD ($)
Lessee, Lease, Description [Line Items]
Letter of credit							$ 1,725	$ 1,258	$ 34
Operating lease, expense							43,400	43,900	$ 34,500
Letter of credit
Lessee, Lease, Description [Line Items]
Letter of credit							32,800	$ 30,000
Seattle, Washington
Lessee, Lease, Description [Line Items]
Lease building space (in sq ft) \| ft²				22,188	22,188	36,126
Annual lease rent per square foot (in dollars per sq ft) \| $ / ft²				62.80	62.80	54
Lease rent, annual increase percentage				2.50%	2.50%	2.50%
Lessee, operating lease, term of contract extension					16 months
Lease extension terms					5 years
Lessee, operating lease, number of contracts \| lease_term					2
60 Binney Street lease
Lessee, Lease, Description [Line Items]
Lease building space (in sq ft) \| ft²		253,108
Annual lease rent per square foot (in dollars per sq ft) \| $ / ft²		72.50	90
Lease payments base annual rent		$ 18,400	$ 22,800
Lease rent, annual increase percentage		1.75%	3%
Collateralized letter of credit		$ 13,800
Decrease in letter of credit under the terms of the lease		9,200
Landlord contribution for cost of construction and tenant improvements		$ 42,400	$ 19,100
60 Binney Street lease \| Letter of credit
Lessee, Lease, Description [Line Items]
Letter of credit			25,000
60 Binney Street lease \| Forecast
Lessee, Lease, Description [Line Items]
Annual lease rent per square foot (in dollars per sq ft) \| $ / ft²	118.41
Lease payments base annual rent	$ 30,000
Lease rent, annual increase percentage	3%
Office and Laboratory Space Lease \| Seattle, Washington \| Letter of credit
Lessee, Lease, Description [Line Items]
Letter of credit			$ 5,000
Construction-in-progress \| Cambridge, Massachusetts
Lessee, Lease, Description [Line Items]
Reimbursement received							$ 18,700

Leases - Summary of Lease Costs

Leases - Summary of Lease Costs and Other Information Pertaining to Operating Leases (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Leases [Abstract]
Operating lease cost	$ 32,636	$ 33,738	$ 24,838
Total lease cost	32,636	33,738	24,838
Operating cash flows used for operating leases	$ 27,927	$ 27,140	$ 25,489
Weighted average remaining lease term	9 years 10 months 24 days	10 years 9 months 18 days	11 years 9 months 18 days
Weighted average discount rate	5.48%	5.47%	5.47%
Variable lease, cost	$ 0	$ 0	$ 0
Short-term lease, cost	$ 0	$ 0	$ 0

Leases - Schedule of Future Min

Leases - Schedule of Future Minimum Commitments Under Operating Leases (Details) $ in Thousands	Dec. 31, 2023 USD ($)
Leases [Abstract]
2024	$ 28,672
2025	29,498
2026	32,920
2027	35,086
2028 and thereafter	215,296
Total lease payments	341,472
Less: imputed interest	(84,799)
Total operating lease liabilities	$ 256,673

Commitments and contingencies -

Commitments and contingencies - Narrative (Detail) - USD ($) $ in Millions	1 Months Ended	3 Months Ended
	Jul. 31, 2021	Jun. 30, 2023	Jun. 30, 2014
Resilience
Commitments And Contingencies Disclosure [Line Items]
Net operating losses, percentage of reimbursement	50%
Reimbursement cap	$ 15
Portion of net operating losses paid		$ 14.2
Pregenen
Commitments And Contingencies Disclosure [Line Items]
Contingent future cash payments			$ 99.9

Commitments and contingencies_2

Commitments and contingencies - Schedule of Non-cancelable Contractual Obligations (Detail) $ in Thousands	Dec. 31, 2023 USD ($)
Commitments and Contingencies Disclosure [Abstract]
2024	$ 6,671
2025 and thereafter	0
Total purchase commitments	$ 6,671

Stockholders' equity - Narrativ

Stockholders' equity - Narrative (Details) $ / shares in Units, $ in Thousands		1 Months Ended					12 Months Ended
	Dec. 13, 2021 shares	Mar. 31, 2023 USD ($) $ / shares shares	Jan. 31, 2023 USD ($) $ / shares shares	Nov. 30, 2022 USD ($)	Mar. 31, 2022 USD ($) $ / shares shares	Aug. 31, 2018 USD ($) $ / shares	Dec. 31, 2023 USD ($) vote shares	Dec. 31, 2022 USD ($) shares	Dec. 31, 2021 USD ($)	Nov. 30, 2021 $ / shares shares	Nov. 03, 2021 shares
Subsidiary, Sale of Stock [Line Items]
Common stock, shares authorized (in shares)							200,000,000	200,000,000
Common stock, number of votes (in votes per share) \| vote							1
Common stock, shares issued (in shares)							50,632,000	37,928,000
Common stock, shares outstanding (in shares)							50,632,000	37,928,000			0
Proceeds from issuance of common stock to Regeneron, net of issuance costs \| $							$ 9,859	$ 0	$ 0
Common stock \| $							$ 5	$ 4
Preferred stock, shares authorized (in shares)							10,000,000	10,000,000
Preferred stock, shares issued (in shares)							0	0
Preferred stock, shares outstanding (in shares)							0	0
Regeneron Collaboration Agreement
Subsidiary, Sale of Stock [Line Items]
Contract with customer liability \| $							$ 4,600
Regeneron Collaboration Agreement \| Share purchase agreement
Subsidiary, Sale of Stock [Line Items]
Common stock price per share (in dollars per share) \| $ / shares			$ 17.94			$ 238.10
Proceeds from issuance of common stock to Regeneron, net of issuance costs \| $			$ 20,000			$ 100,000
Common stock shares issued (in shares)	100,000		1,114,827
Common stock \| $			$ 9,900
Contract with customer liability \| $			$ 10,100
Private Placement
Subsidiary, Sale of Stock [Line Items]
Common stock, shares issued (in shares)					13,934,427
Common stock price per share (in dollars per share) \| $ / shares					$ 12.20
Proceeds from issuance of common stock to Regeneron, net of issuance costs \| $					$ 165,500
Private Placement \| Bluebird Bio
Subsidiary, Sale of Stock [Line Items]
Common stock, shares issued (in shares)										23,368,988
ATM Facility \| Cowen and Company, LLC
Subsidiary, Sale of Stock [Line Items]
Aggregate gross proceeds (up to) \| $				$ 150,000
Underwritten Public Offering
Subsidiary, Sale of Stock [Line Items]
Common stock price per share (in dollars per share) \| $ / shares		$ 11.50
Shares issued in private placement offering (in shares)		10,869,566
Gross proceeds of private placement offering \| $		$ 117,000
Additional shares available for purchase (in shares)		1,630,434
Pre-funded Warrants \| Private Placement
Subsidiary, Sale of Stock [Line Items]
Pre-funded warrants issued (in shares)										757,575
Exercise price of warrants (in dollars per share) \| $ / shares										$ 0.0001

Stockholders' equity - Common S

Stockholders' equity - Common Stock Reserved for Future Issuance (Details) - shares shares in Thousands	Dec. 31, 2023	Dec. 31, 2022
Subsidiary, Sale of Stock [Line Items]
Common stock reserved for future issuance (in shares)	8,098	6,689
Pre-funded Warrants
Subsidiary, Sale of Stock [Line Items]
Common stock reserved for future issuance (in shares)	758	758
2021 Stock Option and Incentive Plan
Subsidiary, Sale of Stock [Line Items]
Common stock reserved for future issuance (in shares)	871	1,621
2021 Employee Stock Purchase Plan
Subsidiary, Sale of Stock [Line Items]
Common stock reserved for future issuance (in shares)	552	434
Options to purchase common stock
Subsidiary, Sale of Stock [Line Items]
Common stock reserved for future issuance (in shares)	3,678	2,562
Restricted stock units
Subsidiary, Sale of Stock [Line Items]
Common stock reserved for future issuance (in shares)	2,239	1,314

Collaborative arrangements an_3

Collaborative arrangements and strategic partnerships - Narrative (Detail)

1 Months Ended

3 Months Ended

12 Months Ended

Dec. 13, 2021 shares

Jan. 31, 2023 USD ($) $ / shares shares

Aug. 31, 2018 USD ($) target accounting_unit $ / shares shares

Sep. 30, 2017 USD ($)

Feb. 29, 2016 USD ($)

Jun. 30, 2015 USD ($)

Mar. 31, 2013 USD ($)

Jun. 30, 2019 USD ($)

Dec. 31, 2023 USD ($) shares

Dec. 31, 2022 USD ($)

Dec. 31, 2021 USD ($)

Apr. 30, 2023 USD ($)

Oct. 27, 2022 USD ($)

Dec. 23, 2021 USD ($) performance_obligation

May 31, 2020 USD ($)

Mar. 31, 2018 USD ($)

Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]

Investment in common stock

$ 9,859,000

$ 0

Deferred revenue, current portion

15,403,000

3,000,000

Deferred revenue, net of current portion

3,918,000

5,000,000

Common stock

5,000

4,000

Bristol-Myers Squibb

Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]

Contract with customer liability

Term of collaboration agreement

3 years

Additions

Bristol-Myers Squibb | Research and development services

Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]

Collaboration agreement, transaction price

40,900,000

Bristol-Myers Squibb | bb21217 research and development services

Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]

Collaboration agreement, transaction price

5,400,000

Bristol-Myers Squibb | BMS collaborative arrangement

Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]

Contract with customer liability

$ 75,000,000

Gross profit

50,000,000

3,100,000

9,400,000

Bristol-Myers Squibb | Amended collaborative agreement

Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]

Contract with customer liability

$ 25,000,000

Term of collaboration agreement

3 years

Bristol-Myers Squibb | Ide-cel license agreement | First product candidates

Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]

Collaboration agreement, option fee received

$ 10,000,000

Bristol-Myers Squibb | Ide-cel license agreement | First product candidates | U.S.

Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]

Milestone payments receivable

85,000,000

Bristol-Myers Squibb | Ide-cel license agreement | First product candidates | Outside of U.S.

Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]

Milestone payments receivable

$ 55,000,000

Bristol-Myers Squibb | Ide-cel co development co promote and profit share agreement

Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]

Contract with customer development milestone payment achieved

$ 10,000,000

Bristol-Myers Squibb | Ide-cel co development co promote and profit share agreement | Outside of U.S.

Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]

Milestone payments receivable

$ 60,000,000

Bristol-Myers Squibb | Ide-cel co development co promote and profit share agreement | Maximum

Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]

Development milestone payments receivable

$ 10,000,000

Bristol-Myers Squibb | Amended Ide-cel co development, co-promote and profit share agreement

Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]

Milestone and royalty obligation buy-out

$ 200,000,000

Bristol-Myers Squibb | bb21217 license agreement

Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]

Collaboration agreement, option fee received

$ 15,000,000

Bristol-Myers Squibb | bb21217 license agreement | U.S.

Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]

Milestone payments receivable

85,000,000

Bristol-Myers Squibb | bb21217 license agreement | Outside of U.S.

Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]

Milestone payments receivable

$ 55,000,000

Bristol-Myers Squibb | Ide-cel research and development

Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]

Revenue

Bristol-Myers Squibb | Ide-cel license and manufacturing services

Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]

Collaboration agreement, transaction price

347,000,000

Bristol-Myers Squibb | Ide-cel license and manufacturing services | License and manufacturing services | Outside of U.S.

Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]

Revenue

14,751,000

13,226,000

16,895,000

Bristol-Myers Squibb | bb21217 research and development services | Phase I, initial obligation

Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]

Revenue

Bristol-Myers Squibb | bb21217 license and manufacturing services | License and manufacturing services

Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]

Contract with customer liability

35,800,000

Revenue

35,762,000

Allocation of Transaction Price

35,500,000

Remaining performance obligation not yet received

1,800,000

Bristol-Myers Squibb | B B21217 License Agreement | U.S.

Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]

Additional fee receivable if option to co-develop and co-promote is not exercised

10,000,000

Regeneron Collaboration Agreement

Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]

Contract with customer liability

4,600,000

Number of initial collaboration targets | target

Research collaboration term

5 years

Number of accounting units | accounting_unit

Joint research activities remaining to be recognized

3,700,000

Regeneron Collaboration Agreement | Collaboration

Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]

Deferred revenue balance recognized as gross revenues

21,600,000

19,600,000

7,500,000

Regeneron Collaboration Agreement | Maximum

Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]

Milestone payments receivable

$ 130,000,000

Regeneron Collaboration Agreement | Research and development services

Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]

Contract with customer liability

1,100,000

Collaboration agreement, transaction price

100,000,000

Purchase price premium

$ 37,000,000

Collaborative arrangement amortization period

5 years

Collaborative arrangement amount attributed to equity sold

$ 54,500,000

Collaborative arrangement amount attributed to joint research activities

$ 45,500,000

Collaborative Arrangement Amount Attributed To Joint Research Activities, Net Funding Advancement

$ 8,500,000

Regeneron Collaboration Agreement | Share purchase agreement

Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]

Contract with customer liability

$ 10,100,000

Issuance of common stock to Regeneron (in shares) | shares

100,000

1,114,827

Common stock price per share (in dollars per share) | $ / shares

$ 17.94

$ 238.10

Investment in common stock

$ 20,000,000

$ 100,000,000

Purchase price premium

$ 37,000,000

Collaborative arrangement research initial funding obligation, percentage

50%

Common stock

$ 9,900,000

Regeneron Collaboration Agreement | Share purchase agreement | Bluebird Bio

Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]

Issuance of common stock to Regeneron (in shares) | shares

400,000

Regeneron Collaboration Agreement | Share purchase agreement | Common stock

Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]

Issuance of common stock to Regeneron (in shares) | shares

400,000

Investment in common stock

$ 63,000,000

Regeneron Collaboration Agreement | Regeneron Collaboration Agreement, Amendment One

Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]

Contract with customer liability

$ 8,300,000

Collaborative arrangement research initial funding obligation, percentage

50%

Deferred revenue, current portion

4,400,000

Deferred revenue, net of current portion

3,900,000

Regeneron Collaboration Agreement | Regeneron Collaboration Agreement, Amendment One | Pre-Clinical Costs To Study Combinations

Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]

Collaborative arrangement research initial funding obligation, percentage

75%

Regeneron Collaboration Agreement | Regeneron Collaboration Agreement, Amendment One | Clinical Study Costs Involving Regeneron Agents

Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]

Collaborative arrangement research initial funding obligation, percentage

100%

Regeneron Collaboration Agreement | Regeneron Collaboration Agreement, Amendment One | MUC-16 Development Candidate Nomination

Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]

Milestone payments receivable

$ 2,000,000

Regeneron Collaboration Agreement | Regeneron Collaboration Agreement, Amendment One | MUC-16 IND Acceptance

Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]

Milestone payments receivable

3,000,000

Regeneron Collaboration Agreement | Regeneron Collaboration Agreement, Amendment One | Last Patient Dosed Or Dosing Of The 10th Patient

Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]

Milestone payments receivable

5,000,000

Regeneron Collaboration Agreement | Regeneron Collaboration Agreement, Amendment One | MUC-16 Development Candidate Nomination After Achievement Of IND Acceptance

Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]

Milestone payments receivable

1,000,000

Regeneron Collaboration Agreement | Regeneron Collaboration Agreement, Amendment One | MUC-16

Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]

Milestone payments receivable

$ 4,000,000

Regeneron Collaboration Agreement | Regeneron Collaboration Agreement, Original And Amendment Combined

Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]

Contract with customer liability

11,200,000

Allocation of Transaction Price

11,259,000

Novo Nordisk A/S | Novo Collaboration and License Agreement

Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]

Contract with customer liability

5,000,000

$ 5,000,000

Revenue

5,765,000

$ 6,501,000

$ 0

Number of performance obligations | performance_obligation

Estimated variable consideration

11,700,000

Estimated variable consideration, upfront payment

5,000,000

Estimated variable consideration, research reimbursement

6,700,000

Novo Nordisk A/S | Novo Collaboration and License Agreement | License and manufacturing services

Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]

Milestone payments receivable

$ 26,000,000

Novo Nordisk A/S | Novo Collaboration and License Agreement | Development and Commercialization Milestones

Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]

Milestone payments receivable

72,000,000

Novo Nordisk A/S | Novo Collaboration and License Agreement | Scientific milestones

Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]

Milestone payments receivable

$ 15,000,000

15,000,000

Novo Nordisk A/S | Novo Collaboration and License Agreement | Extension of research plan without achieving scientific milestones

Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]

Milestone payments receivable

$ 9,000,000

Novo Nordisk A/S | Novo Collaboration and License Agreement | Milestone Achievement, Option To In-License Technology From Third Party

Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]

Contract with customer liability

6,000,000

Milestone payments receivable

$ 9,000,000

Novo Nordisk A/S | Novo Collaboration and License Agreement | Milestone Achievement, Option To In-License Technology From Third Party And Upfront Payment

Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]

Contract with customer liability

11,000,000

JW Therapeutics

Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]

Contract with customer liability

$ 600,000

$ 3,000,000

Estimated variable consideration, reimbursement

4,300,000

Estimated variable consideration

$ 7,300,000

Collaborative arrangements an_4

Collaborative arrangements and strategic partnerships - Summary of Total Transaction Price, Allocation of Total Transaction Price to Identified Performance Obligations Under Arrangement and Amount of Transaction Price Unsatisfied (Detail) - Bristol-Myers Squibb - USD ($) $ in Millions	1 Months Ended
	Sep. 30, 2017	Dec. 31, 2023	May 31, 2020
Amended Ide-cel co development, co-promote and profit share agreement
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Milestone and royalty obligation buy-out			$ 200
Ide-cel license and manufacturing services
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Collaboration agreement, transaction price		$ 347
bb21217 license agreement
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Collaboration agreement, option fee received	$ 15

Collaborative arrangements an_5

Collaborative arrangements and strategic partnerships - Summary of Revenue Recognized or Expense Incurred for Joint Ide-cel Development Efforts Related to Combined Unit of Accounting for its License and Vector Manufacturing of Ide-cel (Detail) - USD ($) $ in Thousands	3 Months Ended												12 Months Ended
	Dec. 31, 2023	Sep. 30, 2023	Jun. 30, 2023	Mar. 31, 2023	Dec. 31, 2022	Sep. 30, 2022	Jun. 30, 2022	Mar. 31, 2022	Dec. 31, 2021	Sep. 30, 2021	Jun. 30, 2021	Mar. 31, 2021	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Research and development expense													$ (230,758)	$ (248,735)	$ (252,617)
Ide-cel research and development services
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Research and development expense	$ (7,086)	$ (6,120)	$ (5,652)	$ (4,871)	$ (6,237)	$ (9,252)	$ (5,463)	$ (6,893)	$ (7,835)	$ (5,660)	$ (9,193)	$ (17,358)	(23,729)	(27,845)	(40,046)
U.S. \| Ide Cel revenue services \| License and manufacturing services \| Accounting Standards Update 2014-09 \| Bristol-Myers Squibb
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Revenue													0	0	533
U.S. \| Ide-cel research and development services \| License and manufacturing services \| Bristol-Myers Squibb
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Research and development expense													0	0	0
Outside of U.S. \| Ide-cel license and manufacturing services \| License and manufacturing services \| Bristol-Myers Squibb
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Revenue													$ 14,751	$ 13,226	$ 16,895

Collaborative arrangements an_6

Collaborative arrangements and strategic partnerships - Collaborative Arrangement Revenue (Detail) - USD ($) $ in Thousands	3 Months Ended												12 Months Ended
	Dec. 31, 2023	Sep. 30, 2023	Jun. 30, 2023	Mar. 31, 2023	Dec. 31, 2022	Sep. 30, 2022	Jun. 30, 2022	Mar. 31, 2022	Dec. 31, 2021	Sep. 30, 2021	Jun. 30, 2021	Mar. 31, 2021	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Collaborative arrangement revenue													$ 71,601	$ 32,358	$ 26,921
Share of collaboration loss													0	(9,642)	(10,071)
ABECMA Commercial Activities
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Collaborative arrangement revenue	$ 1,970	$ 536	$ 24,543	$ 22,961	$ 8,717	$ 4,064	$ 0	$ 0	$ 8,818	$ 10,607	$ 0	$ 0	50,010	12,781	19,425
Share of collaboration loss	0	0	0	0	0	0	(4,290)	(5,352)	0	0	(10,071)	0	0	(9,642)	(10,071)
ABECMA Commercial Activities \| Bristol-Myers Squibb
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Reimbursement of costs	604	1,118	1,271	1,380	1,431	1,215	1,641	1,357	917	845	1,695	0	4,373	5,644	3,457
Share of collaboration loss	$ 1,366	$ (582)	$ 23,272	$ 21,581	$ 7,286	$ 2,849	$ (5,931)	$ (6,709)	$ 7,901	$ 9,762	$ (11,766)	$ 0	$ 45,637	$ (2,505)	$ 5,897

Collaborative arrangements an_7

Collaborative arrangements and strategic partnerships - Summary of Amounts Associated with Research and Development Services (Detail) - USD ($) $ in Thousands	3 Months Ended												12 Months Ended
	Dec. 31, 2023	Sep. 30, 2023	Jun. 30, 2023	Mar. 31, 2023	Dec. 31, 2022	Sep. 30, 2022	Jun. 30, 2022	Mar. 31, 2022	Dec. 31, 2021	Sep. 30, 2021	Jun. 30, 2021	Mar. 31, 2021	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Net R&D Expenses													$ (230,758)	$ (248,735)	$ (252,617)
Ide-cel research and development services
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Net R&D Expenses	$ (7,086)	$ (6,120)	$ (5,652)	$ (4,871)	$ (6,237)	$ (9,252)	$ (5,463)	$ (6,893)	$ (7,835)	$ (5,660)	$ (9,193)	$ (17,358)	(23,729)	(27,845)	(40,046)
Bristol-Myers Squibb \| Ide-cel research and development services
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Obligation for share of research and development expenses	(7,237)	(6,980)	(7,195)	(9,461)	(8,197)	(10,672)	(7,418)	(8,118)	(8,780)	(9,398)	(11,559)	(22,110)	(30,873)	(34,405)	(51,847)
Reimbursement of research and development expenses	$ 151	$ 860	$ 1,543	$ 4,590	$ 1,960	$ 1,420	$ 1,955	$ 1,225	$ 945	$ 3,738	$ 2,366	$ 4,752	$ 7,144	$ 6,560	$ 11,801

Collaborative arrangements an_8

Collaborative arrangements and strategic partnerships - Changes in Balances of Company's Receivables and Contract Liabilities (Detail) - Bristol-Myers Squibb $ in Thousands	12 Months Ended
	Dec. 31, 2023 USD ($)
Receivables
Beginning balance	$ 4,537
Additions	46,212
Deductions	(50,749)
Ending balance	0
Contract liabilities:
Beginning balance	0
Additions	0
Deductions	0
Ending balance	$ 0

Collaborative arrangements an_9

Collaborative arrangements and strategic partnerships - Summary of Transaction Price Allocation (Details) - Regeneron Collaboration Agreement - Regeneron Collaboration Agreement, Original And Amendment Combined $ in Thousands	Dec. 31, 2023 USD ($)
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Allocation of Transaction Price	$ 11,259
Unsatisfied Portion of Transaction Price	8,321
MUC16 Mono/Combo & Next Gen Therapies
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Allocation of Transaction Price	1,905
Unsatisfied Portion of Transaction Price	144
MAGE-A4
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Allocation of Transaction Price	178
Unsatisfied Portion of Transaction Price	10
Early Research Target (1)
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Allocation of Transaction Price	8,701
Unsatisfied Portion of Transaction Price	7,743
Early Research Target (2)
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Allocation of Transaction Price	475
Unsatisfied Portion of Transaction Price	$ 424

Royalty and other revenue - Nar

Royalty and other revenue - Narrative (Detail) - Royalty and other revenue - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2021	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
License And Royalty Revenue [Line Items]
Milestone payment		$ 4,642	$ 3,649	$ 6,220
Juno Therapeutics \| Bluebird Bio
License And Royalty Revenue [Line Items]
Milestone payment	$ 2,500

Intangible assets - Schedule of

Intangible assets - Schedule of Finite-Lived Intangible Assets (Details) - USD ($) $ in Thousands	Dec. 31, 2023	Dec. 31, 2022
Finite-Lived Intangible Assets [Line Items]
Cost	$ 38,600	$ 38,600
Accumulated amortization	(32,006)	(31,298)
Net	6,594	7,302
Developed technology
Finite-Lived Intangible Assets [Line Items]
Cost	30,100	30,100
Accumulated amortization	(30,100)	(30,100)
Net	0	0
In-licensed rights
Finite-Lived Intangible Assets [Line Items]
Cost	8,500	8,500
Accumulated amortization	(1,906)	(1,198)
Net	$ 6,594	$ 7,302

Intangible assets - Narrative (

Intangible assets - Narrative (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Finite-Lived Intangible Assets [Line Items]
Amortization expense	$ 0.7	$ 2.6	$ 4.3
In-licensed rights
Finite-Lived Intangible Assets [Line Items]
Expected useful life	12 years

Intangible assets - Schedule _2

Intangible assets - Schedule of Finite-Lived Intangible Assets, Future Amortization Expense (Details) - USD ($) $ in Thousands	Dec. 31, 2023	Dec. 31, 2022
Goodwill and Intangible Assets Disclosure [Abstract]
2024	$ 708
2025	708
2026	708
2027	708
2028	708
2029 and thereafter	3,054
Net	$ 6,594	$ 7,302

Stock-based compensation - Narr

Stock-based compensation - Narrative (Detail) $ in Thousands			3 Months Ended	12 Months Ended			36 Months Ended
	Nov. 04, 2021 USD ($) shares	Feb. 01, 2021 shares	Dec. 31, 2023 USD ($) shares	Dec. 31, 2023 USD ($) shares	Dec. 31, 2022 USD ($) shares	Dec. 31, 2021 USD ($)	Dec. 31, 2023 USD ($) shares	Sep. 30, 2021
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Conversion ratio			0.3333	0.3333			0.3333	0.3333
Incremental fair value of equity award modification	$ 1,300			$ 1,500
Stock-based compensation expense				32,159	$ 41,040	$ 54,629
Intrinsic value of restricted stock units vested				4,500
Performance and service condition based restricted stock unit
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense				600	600	400
Share-based compensation arrangement by share-based payment award, shares issued in period (less than) (in shares) \| shares	17,387
Vested in period (in shares) \| shares							0
Performance and service condition based restricted stock unit \| Bluebird Bio
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation arrangement by share-based payment award, shares issued in period (less than) (in shares) \| shares		27,000
Options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unrecognized stock- based compensation expense related to unvested stock options, restricted stock awards and employee stock purchase plan			$ 14,600	$ 14,600			$ 14,600
Expected weighted-average period related to unvested stock options, restricted stock awards and employee stock purchase plan				2 years 4 months 24 days
Restricted stock units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense				$ 20,607	$ 22,983	$ 21,414
Unrecognized stock- based compensation expense related to unvested stock options, restricted stock awards and employee stock purchase plan			19,000	$ 19,000			19,000
Expected weighted-average period related to unvested stock options, restricted stock awards and employee stock purchase plan				2 years 1 month 6 days
Vested in period (in shares) \| shares				570,000
Performance Restricted Stock Units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unrecognized stock- based compensation expense related to unvested stock options, restricted stock awards and employee stock purchase plan			$ 0	$ 0			$ 0
Expected weighted-average period related to unvested stock options, restricted stock awards and employee stock purchase plan				0 years
2021 Stock Option and Incentive Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation arrangement by share-based payment award, shares issued in period (less than) (in shares) \| shares			200,000
2021 Stock Option and Incentive Plan \| Unrestricted stock awards
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense				$ 6,500
2021 Employee Stock Purchase Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation arrangement by share-based payment award, shares issued in period (less than) (in shares) \| shares				100,000	100,000

Stock-based compensation - Sche

Stock-based compensation - Schedule of Stock-Based Compensation Expense by Classification (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense	$ 32,159	$ 41,040	$ 54,629
Research and development
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense	12,508	18,333	29,746
Selling, general and administrative
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense	$ 19,651	$ 22,707	$ 24,883

Stock-based compensation - Summ

Stock-based compensation - Summary of Stock-Based Compensation Expense by Award Type (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense	$ 32,159	$ 41,040	$ 54,629
Stock options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense	11,217	17,784	26,185
Restricted stock units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense	20,607	22,983	21,414
Employee stock purchase plan and other
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense	$ 335	$ 273	$ 7,030

Stock-based compensation - Assu

Stock-based compensation - Assumptions Used for the Black-Scholes Option-Pricing Model to Determine the Per Share Weighted Average Fair Value for Options Granted (Detail)	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022
Share-Based Payment Arrangement [Abstract]
Expected volatility	80.90%	78.90%
Expected term (in years)	5 years 10 months 24 days	6 years 1 month 6 days
Risk-free interest rate	4.20%	1.80%
Expected dividend yield	0%	0%

Stock-based compensation - Su_2

Stock-based compensation - Summary of Stock Option Activity Under Plan (Detail) - 2Seventy Employee Equity Awards $ / shares in Units, shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2023 USD ($) $ / shares shares
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]
Outstanding at beginning of period (in shares) \| shares	2,285
Granted (in shares) \| shares	2,024
Exercised (in shares) \| shares	0
Cancelled or forfeited (in shares) \| shares	(830)
Outstanding at end of period (in shares) \| shares	3,479
Exercisable at end of period (in shares) \| shares	1,113
Vested and expected to vest at end of period (in shares) \| shares	3,479
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Exercise Price [Abstract]
Outstanding at beginning of period (in dollars per share) \| $ / shares	$ 53.72
Granted (in dollars per share) \| $ / shares	9.52
Exercised (in dollars per share) \| $ / shares	0
Cancelled or forfeited (in dollars per share) \| $ / shares	75.11
Outstanding at end of period (in dollars per share) \| $ / shares	22.88
Exercisable at end of period (in dollars per share) \| $ / shares	47.51
Vested and expected to vest at end of period (in dollars per share) \| $ / shares	$ 22.88
Weighted-average contractual life (in years)
Outstanding at end of period	8 years 3 months 18 days
Exercisable at end of period	6 years 9 months 18 days
Vested and expected to vest at end of period	8 years 3 months 18 days
Aggregate intrinsic value (in thousands)
Outstanding at end of period \| $	$ 232
Exercisable at end of period \| $	0
Vested and expected to vest at end of period \| $	$ 232

Stock-based compensation - Su_3

Stock-based compensation - Summary of Restricted Stock Units (Detail) - Restricted stock units shares in Thousands	12 Months Ended
	Dec. 31, 2023 $ / shares shares
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward]
Unvested balance at beginning of period (in shares) \| shares	1,270
Granted (in shares) \| shares	1,902
Vested (in shares) \| shares	(570)
Forfeited (in shares) \| shares	(371)
Unvested balance at end of period (in shares) \| shares	2,231
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Weighted Average Grant Date Fair Value [Abstract]
Unvested balance at beginning of period (in dollars per share) \| $ / shares	$ 28.31
Granted (in dollars per share) \| $ / shares	12.30
Vested (in dollars per share) \| $ / shares	34.82
Forfeited (in dollars per share) \| $ / shares	19.06
Unvested balance at end of period (in dollars per share) \| $ / shares	$ 14.54

Related-party transactions - Na

Related-party transactions - Narrative (Details) $ in Thousands	2 Months Ended		3 Months Ended	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2021 USD ($)	Jun. 30, 2024	Dec. 31, 2023 USD ($)	Dec. 31, 2022 USD ($)	Dec. 31, 2021 USD ($)	Nov. 03, 2021 agreement
Related Party Transaction [Line Items]
Other income (expense)				$ (237,608)	$ (263,140)	$ (313,948)
Total operating expenses				337,995	354,636	368,470
Number of transition services agreements \| agreement							2
Initial term of transition services agreement							2 years
Other income, net				7,625	6,055	21,647
Accrued expenses and other current liabilities				25,688	54,678
Receivables and other current assets				13,411	20,960
Accounts payable				6,028	7,208
Selling, general and administrative				$ 69,414	79,450	93,506
Bluebird Bio \| Separation Agreement
Related Party Transaction [Line Items]
Other income (expense)		$ (200)			2,800
Total operating expenses		$ 1,800			200
Bluebird Bio \| Transition Services Agreement
Related Party Transaction [Line Items]
Other income, net					10,000	1,400
Operating expenses					1,000	700
Area of sublease, as a percent	30%
Sublease income					4,900	800
Accrued expenses and other current liabilities					100
Receivables and other current assets					$ 1,900
Bluebird Bio \| Transition Services Agreement \| Forecast
Related Party Transaction [Line Items]
Area of sublease, as a percent			23%
Bluebird Bio \| Management Costs and Corporate Support Services
Related Party Transaction [Line Items]
Selling, general and administrative						$ 55,300

Related-party transactions - Im

Related-party transactions - Imputed Charges (Details) - Bluebird Bio $ in Thousands	12 Months Ended
	Dec. 31, 2021 USD ($)
Related Party Transaction [Line Items]
Related party transaction, amounts of transaction	$ 14,767
Imputed charge to bluebird bio for leases
Related Party Transaction [Line Items]
Related party transaction, amounts of transaction	14,833
Imputed charge from bluebird bio for leases
Related Party Transaction [Line Items]
Related party transaction, amounts of transaction	(908)
Imputed charge to bluebird bio for property, plant and equipment
Related Party Transaction [Line Items]
Related party transaction, amounts of transaction	1,891
Imputed charge from bluebird bio for property, plant and equipment
Related Party Transaction [Line Items]
Related party transaction, amounts of transaction	(1,130)
Imputed charge to bluebird bio for intangible assets
Related Party Transaction [Line Items]
Related party transaction, amounts of transaction	82
Other
Related Party Transaction [Line Items]
Related party transaction, amounts of transaction	$ (1)

401(k) Savings plan - Narrative

401(k) Savings plan - Narrative (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Retirement Benefits [Abstract]
Expenses related to 401(k) plan	$ 3.1	$ 3.2	$ 2.4

Income taxes - Schedule of Comp

Income taxes - Schedule of Components of Loss Before Income Taxes (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Income Tax Disclosure [Abstract]
Domestic	$ (217,570)	$ (254,153)	$ (292,213)
Foreign	0	0	0
Net loss	$ (217,570)	$ (254,153)	$ (292,213)

Income taxes - Reconciliation o

Income taxes - Reconciliation of Income Tax Provision (Benefit) (Detail)	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Income Tax Disclosure [Abstract]
Federal income tax expense at statutory rate	21%	21%	21%
State income tax, net of federal benefit	6%	6.60%	0.60%
Permanent differences	(0.20%)	(0.20%)	0%
Goodwill	(1.20%)	0%	0%
Stock-based compensation	(9.20%)	(2.50%)	0.30%
Research and development credit	5.30%	3.20%	0.20%
Officer compensation limitation	4%	(1.20%)	(0.40%)
Uncertain tax positions	(0.40%)	(0.20%)	0%
Other	0.70%	(0.60%)	(1.10%)
Change in tax rate	(1.80%)	1.30%	0%
Change in valuation allowance	(24.20%)	(27.40%)	64.80%
Separation adjustment	0%	0%	(85.40%)
Effective income tax rate (expense) benefit	0%	0%	0%

Income taxes - Components of De

Income taxes - Components of Deferred Tax Assets and Liabilities (Detail) - USD ($) $ in Thousands	Dec. 31, 2023	Dec. 31, 2022
Deferred tax assets:
U.S. net operating loss carryforwards (federal and state)	$ 38,685	$ 27,251
Tax credit carryforwards (federal and state)	24,933	10,071
Capitalized license fees and research and development expenses	10,562	11,888
Deferred revenue	5,015	3,237
Stock-based compensation	11,834	16,719
Lease liabilities	67,222	73,719
Accruals and other	2,336	3,857
Capitalized research and development expenses	88,721	55,719
Total deferred tax assets	249,308	202,461
Right-of-use assets	(57,087)	(65,181)
Fixed assets	(11,499)	(9,266)
Less: valuation allowance	(180,722)	(128,014)
Net deferred taxes	$ 0	$ 0

Income taxes - Narrative (Detai

Income taxes - Narrative (Detail)	12 Months Ended
Income taxes - Narrative (Detail)	Dec. 31, 2023 USD ($)
Components Of Income Tax Expense Benefit [Line Items]
Approximately valuation allowance increased	$ 52,700,000
Income tax examination, penalties and interest accrued	0
Capitalized research and development expense	$ 203,400,000
Research Tax Credit Carryforward
Components Of Income Tax Expense Benefit [Line Items]
Limitations on use of net operating losses and tax credit carryforwards, percentage	50%
Limitations on use of net operating losses and tax credit carryforwards, period	3 years
U.S.
Components Of Income Tax Expense Benefit [Line Items]
Operating loss carryforwards	$ 143,400,000
U.S. \| Research Tax Credit Carryforward
Components Of Income Tax Expense Benefit [Line Items]
Tax credit carryforward amount	19,700,000
State and Local Jurisdiction
Components Of Income Tax Expense Benefit [Line Items]
Operating loss carryforwards	139,600,000
State and Local Jurisdiction \| Research Tax Credit Carryforward
Components Of Income Tax Expense Benefit [Line Items]
Tax credit carryforward amount	$ 8,900,000

Income taxes - Reconciliation_2

Income taxes - Reconciliation of Unrecognized Tax Benefits (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Balance as of beginning of period	$ 749	$ 36
Increases (decreases) for tax positions related to current period	1,051	713
Increases (decreases) for tax positions as part of separation adjustment	0	0
Balance as of end of period	$ 1,800	$ 749

Net loss per share - Common Sto

Net loss per share - Common Stock Equivalents Excluded from the Calculation of Diluted Net Loss per Share (Details) - shares shares in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Common stock equivalents excluded from the calculation of diluted net loss per share (in shares)	5,998	3,911	2,570
Outstanding stock options
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Common stock equivalents excluded from the calculation of diluted net loss per share (in shares)	3,678	2,562	1,490
Restricted stock units
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Common stock equivalents excluded from the calculation of diluted net loss per share (in shares)	2,239	1,314	1,080
Employee Stock Purchase Plan
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Common stock equivalents excluded from the calculation of diluted net loss per share (in shares)	81	35	0

Corporate restructuring - Narra

Corporate restructuring - Narrative (Details) - USD ($) $ in Thousands	1 Months Ended	12 Months Ended
	Sep. 30, 2023	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Restructuring Cost and Reserve [Line Items]
Restructuring expenses		$ 8,614	$ 0	$ 0
August 2023 Reduction
Restructuring Cost and Reserve [Line Items]
Workforce reduction percentage	40%
Restructuring expenses		$ 8,600

Corporate restructuring - Summa

Corporate restructuring - Summary of Accrued Liability Activity (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Restructuring Reserve [Roll Forward]
Beginning balance	$ 0
Total estimated expenses	8,614	$ 0	$ 0
Expenses paid from inception through December 31, 2023	(6,253)
Reversal of excess accrual through December 31, 2023	0
Ending balance	$ 2,361	$ 0

Goodwill impairment - Narrative

Goodwill impairment - Narrative (Details) $ in Thousands	3 Months Ended	12 Months Ended
Goodwill impairment - Narrative (Details) $ in Thousands	Sep. 30, 2023 USD ($)	Dec. 31, 2023 USD ($) reporting_unit segment	Dec. 31, 2022 USD ($)	Dec. 31, 2021 USD ($)
Goodwill and Intangible Assets Disclosure [Abstract]
Goodwill		$ 0	$ 12,056
Number of operating segments \| segment		1
Number of reporting units \| reporting_unit		1
Goodwill impairment charge	$ 12,100	$ 12,056	$ 0	$ 0

Goodwill impairment - Summary o

Goodwill impairment - Summary of Goodwill Activity (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Sep. 30, 2023	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Goodwill [Roll Forward]
Balance at December 31, 2022		$ 12,056
Additions		0
Deductions	$ (12,100)	(12,056)	$ 0	$ 0
Balance at December 31, 2023		$ 0	$ 12,056

Subsequent events (Details)

Subsequent events (Details) - Subsequent Event $ in Thousands	Jan. 29, 2024 USD ($)
Subsequent Event [Line Items]
Workforce reduction percentage	14%
Disposal Group, Disposed of by Sale, Not Discontinued Operations \| Regeneron Purchase Agreement
Subsequent Event [Line Items]
Consideration received	$ 5,000
Milestone payment upon regulatory approval	10,000
Minimum
Subsequent Event [Line Items]
Expected one-time cash costs	8,000
Maximum
Subsequent Event [Line Items]
Expected one-time cash costs	$ 10,000