Sigilon Therapeutics (SGTX) SC 13GSigilon Therapeutics / Flagship Ventures Fund V ownership change

UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

WASHINGTON, D.C. 20549

SCHEDULE 13G

Under the Securities Exchange Act of 1934

(Amendment No.     )*

Sigilon Therapeutics, Inc.

(Name of Issuer)

Common Stock, $0.001 par value per share

(Title of Class of Securities)

82657L 10 7

(CUSIP Number)

December 31, 2020

(Date of Event Which Requires Filing of this Statement)

Check the appropriate box to designate the rule pursuant to which this Schedule is filed:

☐ Rule 13d-1(b)

☐ Rule 13d-1(c)

☒ Rule 13d-1(d)

The information required in the remainder of this cover page shall not be deemed to be “filed” for the purpose of Section 18 of the Securities Exchange Act of 1934 (“Act”) or otherwise subject to the liabilities of that section of the Act but shall be subject to all other provisions of the Act (however, see the Notes).

Sigilon Therapeutics, Inc. (the “Issuer”)

100 Binney Street

Cambridge, MA 02142

The names of the persons filing this report (collectively, the “Reporting Persons”) are:

Flagship Ventures Fund V, L.P. (“Flagship Fund V”)

Flagship Ventures Fund V General Partner LLC (“Flagship V GP”)

Flagship Pioneering Special Opportunities Fund II, L.P. (“Flagship Opportunities Fund II”)

Flagship Pioneering Special Opportunities Fund II General Partner LLC (“Opportunities Fund II GP”)

Flagship Pioneering, Inc. (“Flagship Pioneering”)

Noubar B. Afeyan, Ph.D. (“Dr. Afeyan”)

The address of the principal business office of each of the Reporting Persons is:

c/o Flagship Pioneering Inc.

55 Cambridge Parkway, Suite 800E

Cambridge, Massachusetts 02142

Common Stock, $0.001 par value per share (“Common Stock”).

82657L 10 7

Not applicable.

The information required by this item with respect to each Reporting Person is set forth in Rows 5 through 9 and 11 of the cover page to this Schedule 13G. The ownership percentages reported are based on 31,240,271 shares of outstanding Common Stock, as reported in the Issuer’s Form 424(b)(4) prospectus filed with the Securities and Exchange Commission on December 7, 2020 (after full exercise of the underwriters’ option to purchase additional shares).

Flagship V GP is the general partner of Flagship Fund V, and, as such, may be deemed to beneficially own the shares beneficially owned by Flagship Fund V.

Dr. Afeyan is the manager of Flagship V GP and, as such, may be deemed to beneficially own shares beneficially owned by Flagship V GP.

Opportunities Fund II GP is the general partner of Flagship Opportunities Fund II and, as such, may be deemed to beneficially own the shares beneficially owned by Flagship Opportunities Fund II.

Flagship Pioneering is the manager of Opportunities Fund II GP and, as such, may be deemed to beneficially own the shares beneficially owned by Opportunities Fund II GP.

Dr. Afeyan is the Chief Executive Officer, sole stockholder and director of Flagship Pioneering and, as such, may be deemed to beneficially own shares beneficially owned by Flagship Pioneering.

If this statement is being filed to report the fact that as of the date hereof the Reporting Persons have ceased to be the beneficial owner of more than five percent of the class of securities, check the following ☐.

Not applicable.

Not applicable.

Not applicable.

Not applicable.

Not applicable.

Material Filed as Exhibits.

Exhibit 1 – Agreement regarding filing of joint Schedule 13G.

SIGNATURE

After reasonable inquiry and to the best of its knowledge and belief, each of the undersigned certifies that the information set forth in this statement is true, complete and correct.

Date: February 16, 2021

EXHIBIT 1

AGREEMENT

The persons below hereby agree that the Schedule 13G to which this agreement is attached as an exhibit, as well as all future amendments to such Schedule 13G, shall be filed on behalf of each of them. This agreement is intended to satisfy the requirements of Rule 13d-1(k)(1)(iii) under the Securities Exchange Act of 1934.

Date: February 16, 2021