Sec Form 4 Filing - Casdin Capital, LLC @ 2seventy bio, Inc. - 2025-05-13

FORM 4 Check this box if no longer subject to Section 16. Form 4 or Form 5 obligations may continue. See Instruction 1(b).

UNITED STATES SECURITIES AND EXCHANGE COMMISSION Washington, D.C. 20549 STATEMENT OF CHANGES IN BENEFICIAL OWNERSHIP Filed pursuant to Section 16(a) of the Securities Exchange Act of 1934, Section 17(a) of the Public Utility Holding Company Act of 1935 or Section 30(h) of the Investment Company Act of 1940

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1. Name and Address of Reporting Person Casdin Capital, LLC 2. Issuer Name and Ticker or Trading Symbol 2seventy bio, Inc. [ TSVT] 5. Relationship of Reporting Person(s) to Issuer (Check all applicable) X __ Director _____ 10% Owner _____ Officer (give title below) _____ Other (specify below) (Last) (First) (Middle) 1350 AVENUE OF THE AMERICAS, SUITE 2600 3. Date of Earliest Transaction (MM/DD/YY) 05/13/2025 (Street) NEW YORK, NY10019 4. If Amendment, Date Original Filed (MM/DD/YY) 6. Individual or Joint/Group Filing (Check Applicable Line) _____ Form filed by One Reporting Person __ X __ Form filed by More than One Reporting Person (City) (State) (Zip)

Table I - Non-Derivative Securities Acquired, Disposed of, or Beneficially Owned 1.Title of Security (Instr. 3) 2. Transaction Date (MM/DD/YY) 2A. Deemed Execution Date, if any (MM/DD/YY) 3. Transaction Code (Instr. 8) 4. Securities Acquired (A) or Disposed of (D) (Instr. 3, 4 and 5) 5. Amount of Securities Beneficially Owned Following Reported Transaction(s) (Instr. 3 and 4) 6. Ownership Form: Direct (D) or Indirect (I) (Instr. 4) 7. Nature of Indirect Beneficial Ownership (Instr. 4) Code V Amount (A) or (D) Price Common Stock, par value $0.0001 per share 05/13/2025 U( 1 )( 2 ) 22,625 D $ 5 ( 1 ) ( 2 ) 0 D ( 3 ) Common Stock, par value $0.0001 per share 05/13/2025 U( 1 )( 2 ) 2,000,000 D $ 5 ( 1 ) ( 2 ) 0 I See footnote ( 4 )

Table II - Derivative Securities Acquired, Disposed of, or Beneficially Owned ( e.g. , puts, calls, warrants, options, convertible securities) 1. Title of Derivative Security (Instr. 3) 2. Conversion or Exercise Price of Derivative Security 3. Transaction Date (MM/DD/YY) 3A. Deemed Execution Date, if any (MM/DD/YY) 4. Transaction Code (Instr. 8) 5. Number of Derivative Securities Acquired (A) or Disposed of (D) (Instr. 3, 4, and 5) 6. Date Exercisable and Expiration Date (MM/DD/YY) 7. Title and Amount of Underlying Securities (Instr. 3 and 4) 8. Price of Derivative Security (Instr. 5) 9. Number of Derivative Securities Beneficially Owned Following Reported Transaction(s) (Instr. 4) 10. Ownership Form of Derivative Security: Direct (D) or Indirect (I) (Instr. 4) 11. Nature of Indirect Beneficial Ownership (Instr. 4) Code V (A) (D) Date Exercisable Expiration Date Title Amount or Number of Shares Stock Options (Right to buy) $ 3.93 05/13/2025 D 12,050 ( 5 ) ( 5 ) ( 5 ) Common Stock, par value $0.0001 per share 12,050 ( 5 ) 0 D ( 3 ) Stock Options (Right to buy) $ 4.54 05/13/2025 D 33,400 ( 5 ) ( 5 ) ( 5 ) Common Stock, par value $0.0001 per share 33,400 ( 5 ) 0 D ( 3 )

Reporting Owners Reporting Owner Name / Address Relationships Director 10% Owner Officer Other Casdin Capital, LLC 1350 AVENUE OF THE AMERICAS SUITE 2600 NEW YORK, NY10019 X Casdin Eli 1350 AVENUE OF THE AMERICAS SUITE 2600 NEW YORK, NY10019 X Casdin Partners Master Fund, L.P. 1350 AVENUE OF THE AMERICAS SUITE 2600 NEW YORK, NY10019 X Casdin Partners GP, LLC 1350 AVENUE OF THE AMERICAS SUITE 2600 NEW YORK, NY10019 X Signatures Casdin Capital LLC, By: /s/ Eli Casdin, Managing Member 05/15/2025 Signature of Reporting Person Date /s/ Eli Casdin, Eli Casdin 05/15/2025 Signature of Reporting Person Date Casdin Partners Master Fund, LP, By: Casdin Partners GP, LLC, its General Partner, By: /s/ Eli Casdin, Managing Member 05/15/2025 Signature of Reporting Person Date Casdin Partners GP LLC, By: /s/ Eli Casdin, Managing Member 05/15/2025 Signature of Reporting Person Date

Explanation of Responses:

( 1 )Represents securities that were tendered in connection with the Agreement and Plan of Merger (the "Merger Agreement"), dated March 10, 2025, by and among 2seventy bio, Inc. (the "Company"), Daybreak Merger Sub Inc. ("Merger Sub") and Bristol-Myers Squibb Company ("Parent"). Following completion of a cash tender offer by Merger Sub to acquire all of the issued and outstanding shares of the Company's common stock, par value $0.0001 per share (the "Company Common Stock") for $5.00 per share (the "Merger Consideration"), Merger Sub merged with and into the Company, with the Company continuing as the surviving corporation and a whole owned subsidiary of Parent (the "Merger"). ( 2 )cont'd from footnote (1): Each restricted stock unit in respect of Company Common Stock ("Company RSU Award"), whether vested or unvested, that was outstanding immediately prior to the effective time of the Merger (the "Effective Time") was fully vested, was cancelled and automatically converted into the right to receive, for each share of Company Common Stock underlying such Company RSU Award immediately prior to the Effective Time, an amount (without interest and subject to deduction for any required withholding under applicable law relating to tax) in cash equal to the Merger Consideration. Additionally, effective as of the Effective Time and as a result of the Merger, Eli Casdin ceased to be a member of the Company's board of directors. ( 3 )The securities were owned directly by Eli Casdin. ( 4 )The securities were owned directly by Casdin Partners Master Fund, L.P. (the "Master Fund") and may be deemed to have been indirectly beneficially owned by (i) Casdin Capital, LLC, the investment adviser to the Master Fund ("Casdin"), (ii) Casdin Partners GP, LLC, the general partner of the Master Fund (the "GP"), and (iii) Eli Casdin, the managing member of Casdin and the GP. ( 5 )Each option to purchase Company Common Stock ("Company Option"), whether vested or unvested, that was outstanding and unexercised immediately prior to the Effective Time and had a per share exercise price that was less than the Merger Consideration was fully vested, was cancelled and automatically converted into the right to receive, for each share of Company Common Stock underlying such Company Option immediately prior to the Effective Time, an amount (without interest and subject to deduction for any required withholding under applicable law relating to tax) in cash equal to the excess of the Merger Consideration over the per share exercise price of such Company Option.

Remarks: Each Reporting Person disclaims beneficial ownership in the securities reported on this Form 4 except to the extent of its pecuniary interest, if any, therein, and this report shall not be deemed to be an admission that such Reporting Person is the beneficial owner of such securities for purposes of Section 16 or for any other purpose.

Reminder: Report on a separate line for each class of securities beneficially owned directly or indirectly.

* If the form is filed by more than one reporting person, see Instruction 4(b)(v).

** Intentional misstatements or omissions of facts constitute Federal Criminal Violations. See 18 U.S.C. 1001 and 15 U.S.C. 78ff(a).

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