Sec Form 4 Filing - ARCH Venture Partners X, LLC @ Prime Medicine, Inc. - 2022-10-24

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 ARCH Venture Partners X, LLC 2. Issuer Name and Ticker or Trading Symbol Prime Medicine, Inc. [ PRME] 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) 8755 WEST HIGGINS ROAD, SUITE 1025 3. Date of Earliest Transaction (MM/DD/YY) 10/24/2022 (Street) CHICAGO, IL60631 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 10/24/2022 C 5,728,297 A 5,728,297 I By ARCH Venture Fund X, L.P.( 2 )( 3 ) Common Stock 10/24/2022 C 5,728,297 A 5,728,297 I By ARCH Venture Fund X Overage, L.P.( 3 )( 4 ) Common Stock 10/24/2022 P 400,000 A $ 17 6,128,297 I By ARCH Venture Fund X, L.P.( 2 )( 3 )( 5 ) Common Stock 10/24/2022 P 400,000 A $ 17 6,128,297 I By ARCH Venture Fund X Overage, L.P.( 2 )( 3 )( 6 )

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 Convertible Preferred Stock ( 1 ) 10/24/2022 C 16,666,667 ( 1 ) ( 1 ) Common Stock 5,361,122 $ 0 0 I By ARCH Venture Fund X, L.P( 2 )( 3 ) Series A Convertible Preferred Stock ( 1 ) 10/24/2022 C 16,666,664 ( 1 ) ( 1 ) Common Stock 5,361,122 $ 0 0 I By ARCH Venture Fund X Overage, L.P.( 3 )( 4 ) Series B Convertible Preferred Stock ( 1 ) 10/24/2022 C 1,141,474 ( 1 ) ( 1 ) Common Stock 367,175 $ 0 0 I By ARCH Venture Fund X, L.P.( 2 )( 3 ) Series B Convertible Preferred Stock ( 5 ) 10/24/2022 C 1,141,474 ( 5 ) ( 5 ) Common Stock 367,175 $ 0 0 I By ARCH Venture Fund X Overage, L.P.( 3 )( 4 )

Reporting Owners Reporting Owner Name / Address Relationships Director 10% Owner Officer Other ARCH Venture Partners X, LLC 8755 WEST HIGGINS ROAD, SUITE 1025 CHICAGO, IL60631 X ARCH Venture Partners X, L.P. 8755 WEST HIGGINS ROAD, SUITE 1025 CHICAGO, IL60631 X ARCH Venture Partners X Overage, L.P. 8755 WEST HIGGINS ROAD, SUITE 1025 CHICAGO, IL60631 X ARCH Venture Fund X, L.P. 8755 WEST HIGGINS ROAD, SUITE 1025 CHICAGO, IL60631 X ARCH Venture Fund X Overage, L.P. 8755 WEST HIGGINS ROAD, SUITE 1025 CHICAGO, IL60631 X CRANDELL KEITH 8755 WEST HIGGINS ROAD, SUITE 1025 CHICAGO, IL60631 X Burow Kristina 8755 WEST HIGGINS ROAD, SUITE 1025 CHICAGO, IL60631 X GILLIS STEVEN 8755 WEST HIGGINS ROAD, SUITE 1025 CHICAGO, IL60631 X Signatures /s/ ARCH Venture Fund X, L.P., By: ARCH Venture Partners X, L.P., its General Partner, By: ARCH Venture Partners X, LLC, its General Partner, By: Mark McDonnell, attorney-in-fact 10/26/2022 Signature of Reporting Person Date /s/ ARCH Venture Fund X Overage, L.P., By: ARCH Venture Partners X Overage, L.P., its General Partner, By: ARCH Venture Partners X, LLC, its General Partner, By: Mark McDonnell, attorney-in-fact 10/26/2022 Signature of Reporting Person Date /s/ ARCH Venture Partners X, L.P., By: ARCH Venture Partners X, LLC, its General Partner, By: Mark McDonnell, attorney-in-fact 10/26/2022 Signature of Reporting Person Date /s/ ARCH Venture Partners X Overage, L.P., By: ARCH Venture Partners X, LLC, its General Partner, By: Mark McDonnell, attorney-in-fact 10/26/2022 Signature of Reporting Person Date /s/ ARCH Venture Partners X, LLC, By: Mark McDonnell, attorney-in-fact 10/26/2022 Signature of Reporting Person Date /s/ Keith Crandell, By: Mark McDonnell, attorney-in-fact 10/26/2022 Signature of Reporting Person Da te /s/ Kristina Burow, By: Mark McDonnell, attorney-in-fact 10/26/2022 Signature of Reporting Person Date /s/ Steven Gillis, By: Mark McDonnell, attorney-in-fact 10/26/2022 Signature of Reporting Person Date

Explanation of Responses:

( 1 )The shares of Series A Convertible Preferred Stock and Series B Convertible Preferred Stock (together, the "Preferred Stock") were convertible into Common Stock on a one-for-3.10880 basis at any time at the option of the holder, and automatically converted into the number of shares shown in Column 7 immediately prior to the closing of the Issuer's initial public offering on October 24, 2022. The Preferred Stock had no expiration date. ( 2 )Represents shares held directly by ARCH Venture Fund X, L.P. ("ARCH X"). ARCH Venture Partners X, L.P. ("AVP X LP") is the sole general partner of ARCH X. ( 3 )ARCH Venture Partners X, LLC ("AVP X LLC") is the sole general partner of each of AVP X LP and AVP X Overage LP. Keith Crandell, Kristina Burow and Steven Gillis are members of the investment committee of AVP X LLC (the "AVP X Committee Members"). AVP X LP and AVP X Overage LP may be deemed to beneficially own the shares held by ARCH X and ARCH X Overage, respectively, AVP X LLC may be deemed to beneficially own the shares held by ARCH X and ARCH X Overage, and each of the AVP X Committee Members may be deemed to share the power to direct the disposition and vote of the shares held by ARCH X and ARCH X Overage. AVP X LP, AVP X Overage LP, AVP X LLC, and the AVP X Committee Members each disclaim beneficial ownership except to the extent of any pecuniary interest therein, and this report shall not be deemed an admission that such shares are beneficially owned by them for Section 16 or any other purpose. ( 4 )Represents shares held directly by ARCH Venture Fund X Overage, L.P. ("ARCH X Overage"). ARCH Venture Partners X Overage, L.P. ("AVP X Overage LP") is the sole general partner of ARCH X Overage. ( 5 )Reflects shares purchased by ARCH X in the Issuer's initial public offering. ( 6 )Reflects shares purchased by ARCH X Overage in the Issuer's initial public offering.

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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