Sec Form 3 Filing - POWELL MICHAEL @ Checkmate Pharmaceuticals, Inc. - 2020-08-06

FORM 3 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 POWELL MICHAEL 2. Issuer Name and Ticker or Trading Symbol Checkmate Pharmaceuticals, Inc. [ CMPI] 5. Relationship of Reporting Person(s) to Issuer (Check all applicable) X __ Director X __ 10% Owner _____ Officer (give title below) _____ Other (specify below) (Last) (First) (Middle) C/O SOFINNOVA INVESTMENTS, INC., 3000 SAND HILL ROAD, BLDG. 4, SUITE 250 3. Date of Earliest Transaction (MM/DD/YY) 08/06/2020 (Street) MENLO PARK, CA94025 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

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 Preferred Stock ( 1 ) ( 1 ) ( 1 ) Common Stock 1,671,771 I See footnote ( 3 ) Series B Preferred Stock ( 2 ) ( 2 ) ( 2 ) Common Stock 1,314,165 I See footnote ( 3 ) Series C Preferred Stock ( 1 ) ( 1 ) ( 1 ) Common Stock 487,438 I See footnote ( 3 )

Reporting Owners Reporting Owner Name / Address Relationships Director 10% Owner Officer Other POWELL MICHAEL C/O SOFINNOVA INVESTMENTS, INC. 3000 SAND HILL ROAD, BLDG. 4, SUITE 250 MENLO PARK, CA94025 X X Sofinnova Venture Partners IX, L.P. C/O SOFINNOVA INVESTMENTS, INC. 3000 SAND HILL ROAD, BLDG. 4, SUITE 250 MENLO PARK, CA94025 X Sofinnova Management IX, L.L.C. C/O SOFINNOVA INVESTMENTS, INC. 3000 SAND HILL ROAD, BLDG. 4, SUITE 250 MENLO PARK, CA94025 X HEALY JAMES C/O SOFINNOVA INVESTMENTS, INC. 3000 SAND HILL ROAD, BLDG. 4, SUITE 250 MENLO PARK, CA94025 X Signatures /s/ Nathalie Auber, attorney-in-fact for Sofinnova Venture Partners IX, L.P. 08/06/2020 Signature of Reporting Person Date /s/ Nathalie Auber, attorney-in-fact for Sofinnova Management IX, L.L.C.. 08/06/2020 Signature of Reporting Person Date /s/ Nathalie Auber, attorney-in-fact for James I Healy 08/06/2020 Signature of Reporting Person Date /s/ Nathalie Auber, attorney-in-fact for Michael F. Powell 08/06/2020 Signature of Reporting Person Date

Explanation of Responses:

( 1 )The Series A Preferred Stock and Series C Preferred Stock is convertible into Common Stock at a rate of 1:7.4771 into the number of shares of Common Stock as shown in Column 3 at any time at the holder's election, and automatically upon the closing of the Issuer's initial public offering without payment or further consideration. The shares have no expiration date. ( 2 )The Series B Preferred Stock is convertible into Common Stock at a rate of 1.12256:7.4771 into the number of shares of Common Stock as shown in Column 3 at any time at the holder's election, and automatically upon the closing of the Issuer's initial public offering without payment or further consideration. The shares have no expiration date. ( 3 )The shares are held directly by Sofinnova Venture Partners IX, L.P. (the "Fund"). The general partner of the Fund is Sofinnova Management IX, L.L.C. (the "GP") and may be deemed to have sole voting, investment and dispositive power with respect to the securities held by the Fund. Dr. James I. Healy and Michael F. Powell, Ph.D. are the managing members of the GP and may be deemed to have shared voting, investment and dispositive power with respect to the securities held by the Fund. Each reporting person disclaims beneficial ownership of such securities, except to the extent of his or its proportionate pecuniary interest therein, if any.

Remarks: Exhibit 24 - Power of Attorney

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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