Sec Form 4 Filing - MPM ASSET MANAGEMENT LLC @ TCR2 THERAPEUTICS INC. - 2023-05-22

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 MPM ASSET MANAGEMENT LLC 2. Issuer Name and Ticker or Trading Symbol TCR2 THERAPEUTICS INC. [ TCRR] 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 MPM CAPITAL, 399 BOYLSTON STREET, SUITE 1100 3. Date of Earliest Transaction (MM/DD/YY) 05/22/2023 (Street) BOSTON, MA02116 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 05/22/2023 X( 1 ) 178,269 A $ 0.74 374,171 D Common Stock 05/22/2023 S( 1 ) 70,170 D $ 1.88 304,001 D Common Stock 3,370,977 I See Footnote ( 2 )

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 Common Stock Warrant (Right To Buy) $ 0.74 05/22/2023 X( 1 ) 42,761 12/13/2016 12/12/2026 Common Stock 42,761 $ 0 0 D Common Stock Warrant (Right to Buy) $ 0.74 05/22/2023 X( 1 ) 135,508 12/06/2017 12/05/2027 Common Stock 135,508 $ 0 0 D

Reporting Owners Reporting Owner Name / Address Relationships Director 10% Owner Officer Other MPM ASSET MANAGEMENT LLC C/O MPM CAPITAL 399 BOYLSTON STREET, SUITE 1100 BOSTON, MA02116 X EVNIN LUKE C/O MPM CAPITAL 399 BOYLSTON STREET, SUITE 1100 BOSTON, MA02116 X Signatures /s/ Luke Evnin, member of MPM Asset Management LLC 05/24/2023 Signature of Reporting Person Date /s/ Luke Evnin 05/24/2023 Signature of Reporting Person Date

Explanation of Responses:

( 1 )On May 22, 2023, MPM Asset Management LLC ("AM LLC") exercised warrants to purchase 178,269 shares of the Issuer's common stock for $0.74 a share. AM LLC paid the exercise price on a cashless basis, resulting in the Issuer's withholding of 70,170 of the warrant shares to satisfy the exercise price and issuing to AM LLC the remaining 108,099 shares. ( 2 )The shares are held as follows: 3,056,272 by MPM BioVentures 2014, L.P. ("BV 2014"), 203,846 by MPM BioVentures 2014(B), L.P. ("BV 2014(B)") and 110,859 by MPM Asset Management Investors BV2014 LLC ("AM BV2014"). MPM BioVentures 2014 GP LLC and MPM BioVentures 2014 LLC ("BV LLC") are the direct and indirect general partners of BV 2014 and BV 2014(B) and BV LLC is the manager of AM BV 2014. Luke Evnin is a managing director of BV LLC and disclaims beneficial ownership of the reported securities except to the extent of his pecuniary interest therein.

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

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