As filed with the Securities and Exchange Commission on May 13, 2025

Registration No. 333-276403

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UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

WASHINGTON, D.C. 20549

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POST-EFFECTIVE AMENDMENT TO

FORM S-8 REGISTRATION STATEMENT No. 333-276403

FORM S-8 REGISTRATION STATEMENT No. 333-270660

FORM S-8 REGISTRATION STATEMENT No. 333-263853

FORM S-8 REGISTRATION STATEMENT No. 333-260669

UNDER THE SECURITIES ACT OF 1933

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2SEVENTY BIO, INC.

(Exact Name of Registrant As Specified In Its Charter)

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2021 Stock Option and Incentive Plan

2021 Employee Stock Purchase Plan

(Full title of the plan)

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Amy Fallone

Vice President and Secretary

2seventy bio, Inc.

60 Binney St.

Cambridge, MA 02142

(Name and address of agent for service)

(617) 675-7270

(Telephone number, including area code, of agent for service)

Copies to:

Catherine J. Dargan, Esq.

Kerry S. Burke, Esq.

Andrew Fischer, Esq.

Kyle Rabe, Esq.

Covington & Burling LLP

One CityCenter

850 Tenth Street, NW

Washington, DC 20001-4956

(202) 662-6000

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Indicate by check mark whether the registrant is a large accelerated filer, an accelerated filer, a non-accelerated filer, smaller reporting company, or an emerging growth company. See the definitions of “large accelerated filer,” “accelerated filer,” “smaller reporting company,” and “emerging growth company” in Rule 12b-2 of the Exchange Act.

If an emerging growth company, indicate by check mark if the registrant has elected not to use the extended transition period for complying with any new or revised financial accounting standards provided pursuant to Section 7(a)(2)(B) of the Securities Act. ☐

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EXPLANATORY NOTE

This Post-Effective Amendment relates to the following Registration Statements on Form S-8 (each a “Registration Statement” and collectively, the “Registration Statements”), filed with the Securities and Exchange Commission (the “SEC”) by 2seventy bio, Inc., a Delaware corporation (the “Registrant”):

On March 10, 2025, the Registrant entered into that certain Agreement and Plan of Merger (the “Merger Agreement”) with Bristol-Myers Squibb Company, a Delaware corporation ( “Bristol-Myers Squibb”), and Daybreak Merger Sub Inc., a Delaware corporation and a wholly owned subsidiary of Parent (“Merger Sub”). Pursuant to the Merger Agreement, on May 13, 2025, Merger Sub merged with and into the Registrant with the Registrant surviving the merger as a wholly owned subsidiary of Bristol-Myers Squibb (the “Merger”).

In connection with the Merger, the Registrant has terminated, as of the date hereof, any and all of the offerings of the Registrant’s securities pursuant to the Registration Statements. In accordance with the undertakings made by the Registrant in the Registration Statements to remove from registration by means of a post-effective amendment any securities that had been registered for issuance but remain unsold at the termination of the offering, the Registrant hereby removes and withdraws from registration any and all securities registered pursuant to the Registration Statements that remain unsold as of the date hereof, and each Registration Statement is hereby amended, as appropriate, to reflect the deregistration of such securities.

SIGNATURES

Pursuant to the requirements of the Securities Act of 1933, as amended, the Registrant certifies that it has reasonable grounds to believe that it meets all of the requirements for filing on Form S-8 and has duly caused these Post-Effective Amendments to the Registration Statements on Form S-8 to be signed on its behalf by the undersigned, thereunto duly authorized, in the City of Princeton, New Jersey, on May 13, 2025.

No other person is required to sign these Post-Effective Amendments to the Registration Statements in reliance on Rule 478 of the Securities Act of 1933, as amended.

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