Sec Form 4 Filing - Third Rock Ventures V, L.P. @ CARGO Therapeutics, Inc. - 2023-11-14

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 Third Rock Ventures V, L.P. 2. Issuer Name and Ticker or Trading Symbol CARGO Therapeutics, Inc. [ CRGX] 5. Relationship of Reporting Person(s) to Issuer (Check all applicable) _____ Director X __ 10% Owner _____ Officer (give title below) _____ Other (specify below) (Last) (First) (Middle) C/O THIRD ROCK VENTURES, LLC, 201 BROOKLINE AVE, SUITE 1401 3. Date of Earliest Transaction (MM/DD/YY) 11/14/2023 (Street) BOSTON, MA02215 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 11/14/2023 C 1,737,216 ( 1 ) A 1,737,216 D ( 3 ) ( 4 ) ( 8 ) Common Stock 11/14/2023 C 473,786 ( 5 ) A 473,786 I See Footnotes ( 6 ) ( 7 ) ( 8 ) Common Stock 11/14/2023 P 1,333,333 A $ 15 1,807,119 I See Footnotes ( 6 ) ( 7 ) ( 8 )

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 Series A-1 Preferred Stock ( 2 ) 11/14/2023 C 23,571,429 ( 2 ) ( 2 ) Common Stock 1,737,216 $ 0 0 D ( 3 ) ( 4 ) ( 8 ) Series A-1 Preferred Stock ( 2 ) 11/14/2023 C 6,428,571 ( 2 ) ( 2 ) Common Stock 473,786 $ 0 0 I See Footnotes ( 6 ) ( 7 ) ( 8 )

Reporting Owners Reporting Owner Name / Address Relationships Director 10% Owner Officer Other Third Rock Ventures V, L.P. C/O THIRD ROCK VENTURES, LLC 201 BROOKLINE AVE, SUITE 1401 BOSTON, MA02215 X Third Rock Ventures GP V, LP 201 BROOKLINE AVE, SUITE 1401 BOSTON, MA02215 X TRV GP V, LLC 201 BROOKLINE AVE, SUITE 1401 BOSTON, MA02215 X Third Rock Ventures VI, L.P. 201 BROOKLINE AVE, SUITE 1401 BOSTON, MA02215 X Third Rock Ventures GP VI, L.P. 201 BROOKLINE AVE, SUITE 1401 BOSTON, MA02215 X TRV GP VI, LLC 201 BROOKLINE AVE, SUITE 1401 BOSTON, MA02215 X Signatures /s/ Kevin Gillis, Chief Operating Officer of TRV GP V, LLC, general partner of Third Rock Ventures GP V, L.P., general partner of Third Rock Ventures V, L.P. 11/14/2023 Signature of Reporting Person Date /s/ Kevin Gillis, Chief Operating Officer of TRV GP V, LLC, general partner of Third Rock Ventures GP V, L.P. 11/14/2023 Signature of Reporting Person Date /s/ Kevin Gillis, Chief Operating Officer of TRV GP V, LLC 11/14/2023 Signature of Reporting Person Date /s/ Kevin Gillis, Chief Operating Officer of TRV GP VI, LLC, general partner of Third Rock Ventures GP VI, L.P., general partner of Third Rock Ventures VI, L.P. 11/14/2023 Signature of Reporting Person Date /s/ Kevin Gillis, Chief Operating Officer of TRV GP VI, LLC, general partner of Third Rock Ventures GP VI, L.P. 11/14/2023 Signature of Reporting Person Date /s/ Kevin Gillis, Chief Operating Officer of TRV GP VI, LLC 11/14/2023 Signature of Reporting Person Date

Explanation of Responses:

( 1 )Represents the total number of shares of Common Stock received by Third Rock Ventures V, L.P. ("TRV V") upon conversion of the preferred stock of the Issuer in connection with the closing of the Issuer's initial public offering. ( 2 )The Series A-1 Preferred Stock converted into Common Stock on a 13.5685-for-1 basis upon the closing of the Issuer's initial public offering without payment of consideration. The Series A-1 Preferred Stock was convertible at any time at the holder's election and automatically upon the closing of the Issuer's initial public offering. The Series A-1 Preferred Stock had no expiration date. ( 3 )These shares are directly held by TRV V. ( 4 )The general partner of TRV V is Third Rock Ventures GP V, L.P. ("TRV GP V"). The general partner of TRV GP V is TRV GP V, LLC ("TRV GP V LLC"). Each of TRV GP V and TRV GP V LLC disclaims beneficial ownership of these shares except to the extent of its or his pecuniary interest therein, if any, and this report shall not be deemed an admission that it or he is the beneficial owner of such shares. ( 5 )Represents the total number of shares of Common Stock received by Third Rock Ventures VI, L.P. ("TRV VI") upon conversion of the preferred stock of the Issuer in connection with the closing of the Issuer's initial public offering. ( 6 )These shares are directly held by TRV VI. ( 7 )The general partner of TRV VI is Third Rock Ventures GP VI, L.P. ("TRV GP VI"). The general partner of TRV GP VI is TRV GP VI, LLC ("TRV GP VI LLC"). Each of TRV GP VI and TRV GP VI LLC disclaims beneficial ownership of these shares except to the extent of its pecuniary interest therein, if any, and this report shall not be deemed an admission that it or he is the beneficial owner of such shares. ( 8 )Each of Reporting Persons disclaims the existence of a Section 13(d) "group" as between the TRV V related parties and the TRV VI related parties and this report shall not be deemed an admission that any of such parties is or may be part of such a group with any of the other parties.

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).

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