Solid Biosciences (SLDB) DEFA14AAdditional proxy soliciting materials

UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

SCHEDULE 14A

(Rule 14a-101)

INFORMATION REQUIRED IN PROXY STATEMENT

SCHEDULE 14A INFORMATION

Proxy Statement Pursuant to Section 14(a) of the

Securities Exchange Act of 1934

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Check the appropriate box:

SOLID BIOSCIENCES, INC.

(Name of Registrant as Specified in its Charter)

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Your Vote Counts SOLID BIOSCIENCES INC. 2022 Annual Meeting Vote by June 6, 2022 11:59 PM ET SOLID BIOSCIENCES INC. C/O PROXY SERVICES P.O. BOX 9142 FARMINGDALE, NY 11735 D83413-P72480 You invested in SOLID BIOSCIENCES INC. and it’s time to vote You have the right to vote on proposals being presented at the Annual Meeting. This is an important notice regarding the availability of proxy material for the stockholder meeting to be held on June 7, 2022. Get informed before you vote View the Notice and Proxy Statement and Form 10-K online OR you can receive a free paper or email copy of the material(s) by requesting prior to May 24, 2022. If you would like to request a copy of the material(s) for this and/or future stockholder meetings, you may (1) visit www.ProxyVote.com, (2) call 1-800-579-1639 or (3) send an email to sendmaterial@proxyvote.com. If sending an email, please include your control number (indicated below) in the subject line. Unless requested, you will not otherwise receive a paper or email copy. For complete information and to vote, visit www.ProxyVote.com Control # Smartphone users Vote Virtually at the Meeting* Point your camera here and June 7, 2022 8:00 AM EDT vote without entering a control number Virtually at: www.virtualshareholdermeeting.com/SLDB2022 *Please check the meeting materials for any special requirements for meeting attendance. V1.1

Vote at www.ProxyVote.com THIS IS NOT A VOTABLE BALLOT This is an overview of the proposals being presented at the upcoming stockholder meeting. Please follow the instructions on the reverse side to vote these important matters. Voting Items Board Recommends 1. Election of Class I Directors Nominees: 1a. Robert Huffines 1b. Sukumar Nagendran For 1c. Rajeev Shah 2. The ratification of the appointment of PricewaterhouseCoopers LLP as the Company’s independent registered public accounting firm for the fiscal year ending December 31, 2022. For 3. The approval of an amendment to our certificate of incorporation to effect a reverse stock split at a ratio of not less than 1-for-5 and not greater than 1-for-15, with the exact ratio to be set within that range at the discretion of the board of directors prior to the Company’s 2023 annual meeting of stockholders without further approval or authorization of For our stockholders and with our board of directors able to elect to abandon such proposed amendment and not effect the reverse stock split authorized by stockholders, in its sole discretion, and, in connection therewith, to decrease the number of authorized shares of our common stock on a basis proportional to the reverse stock split ratio. 4. The approval of an amendment to our certificate of incorporation to set the number of authorized shares of our common stock at a number determined by calculating the product of 300,000,000 multiplied by three times (3x) the reverse For stock split ratio, subject to approval by our stockholders of the reverse stock split proposal and our implementation of a reverse stock split as set forth in Proposal 3. NOTE: The proxies are authorized to vote, in their discretion, upon such other business as may properly come before the meeting or any adjournment or postponement thereof. Prefer to receive an email instead? While voting on www.ProxyVote.com, be sure to click “Sign up for E-delivery”. D83414-P72480