Sec Form 4 Filing - Atlas Venture Fund XI, L.P. @ Dyne Therapeutics, Inc. - 2024-02-21

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 Atlas Venture Fund XI, L.P. 2. Issuer Name and Ticker or Trading Symbol Dyne Therapeutics, Inc. [ DYN] 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) 300 TECHNOLOGY SQUARE, 8TH FLOOR 3. Date of Earliest Transaction (MM/DD/YY) 02/21/2024 (Street) CAMBRIDGE, MA02139 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 02/21/2024 S( 1 ) 149,337 D $ 22.44 ( 2 ) 6,666,628 D ( 3 ) Common Stock 02/21/2024 S( 1 ) 35,164 D $ 23.14 ( 4 ) 6,631,464 D ( 3 ) Common Stock 02/22/2024 S( 1 ) 119,360 D $ 22.66 ( 5 ) 6,512,104 D ( 3 ) Common Stock 02/23/2024 S( 1 ) 159,542 D $ 22.7 ( 6 ) 6,352,562 D ( 3 ) Common Stock 02/23/2024 S( 7 ) 23,648 D $ 22.7 ( 6 ) 1,585,137 I See footnote ( 8 ) Common Stock 02/23/2024 S( 9 ) 13,738 D $ 22.7 ( 6 ) 920,843 I See footnote ( 10 )

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

Reporting Owners Reporting Owner Name / Address Relationships Director 10% Owner Officer Other Atlas Venture Fund XI, L.P. 300 TECHNOLOGY SQUARE, 8TH FLOOR CAMBRIDGE, MA02139 X Atlas Venture Associates XI, L.P. 300 TECHNOLOGY SQUARE, 8TH FLOOR CAMBRIDGE, MA02139 X Atlas Venture Associates XI, LLC 300 TECHNOLOGY SQUARE, 8TH FLOOR CAMBRIDGE, MA02139 X Atlas Venture Opportunity Fund I, L.P. 300 TECHNOLOGY SQUARE, 8TH FLOOR CAMBRIDGE, MA02139 X Atlas Venture Associates Opportunity I, L.P. 300 TECHNOLOGY SQUARE, 8TH FLOOR CAMBRIDGE, MA02139 X Atlas Venture Associates Opportunity I, LLC 300 TECHNOLOGY SQUARE, 8TH FLOOR CAMBRIDGE, MA02139 X Atlas Venture Opportunity Fund II, L.P. 300 TECHNOLOGY SQUARE, 8TH FLOOR CAMBRIDGE, MA02139 X Atlas Venture Associates Opportunity II, LP 300 TECHNOLOGY SQUARE, 8TH FLOOR CAMBRIDGE, MA02139 X Atlas Venture Associates Opportunity II, LLC 300 TECHNOLOGY SQUARE, 8TH FLOOR CAMBRIDGE, MA02139 X Signatures Atlas Venture Fund XI, LP, By: Atlas Venture Associates XI, L.P., its general partner, By: Atlas Venture Associates XI, LLC, its general partner, By: /s/ Ommer Chohan, Chief Financial Officer 02/23/2024 Signature of Reporting Person Date Atlas Venture Associates XI, L.P., By: Atlas Venture Associates XI, LLC, its general partner, By: /s/ Ommer Chohan, Chief Financial Officer 02/23/2024 Signature of Reporting Person Date Atlas Venture Associates XI, LLC By: /s/ Ommer Chohan, Chief Financial Officer 02/23/2024 Signature of Reporting Person Date Atlas Venture Opportunity Fund I, L.P., By: Atlas Venture Associates Opportunity I, L.P., its general partner, By: Atlas Venture Associates Opportunity I, LLC, its general partner, By: /s/ Ommer Chohan, Chief Financial Officer 02/23/2024 Signature of Reporting Person Date Atlas Venture Associates Opportunity I, L.P. By: Atlas Venture Associates Opportunity I, LLC, its general partner, By: /s/ Ommer Chohan, Chief Financial Officer 02/23/2024 Signature of Reporting Person Date Atlas Venture Associates Opportunity I, LLC, By: /s/ Ommer Chohan, Chief Financial Officer 02/23/2024 Signature of Reporting Person Date Atlas Venture Opportunity Fund II, L.P., By: Atlas Venture Associates Opportunity II, L.P., its general partner, By: Atlas Venture Associates Opportunity II, LLC, its general partner, By: /s/ Ommer Chohan, Chief Financial Officer 02/23/2024 Signature of Reporting Person Date /s/ Atlas Venture Associates Opportunity II, L.P., By: Atlas Venture Associates Opportunity II, LLC, its general partner, By: /s/ Ommer Chohan, Chief Financial Officer 02/23/2024 Signature of Reporting Person Date Atlas Venture Associates Opportunity II, LLC, By: /s/ Ommer Chohan, Chief Financial Officer 02/23/2024 Signature of Reporting Person Date

Explanation of Responses:

( 1 )This transaction was made pursuant to a Rule 10b5-1 trading plan adopted by Atlas Venture Fund XI, L.P. on January 10, 2024. ( 2 )The price reported in Column 4 is a weighted average p rice. These shares were sold in multiple transactions at prices ranging from $21.75 to $22.72 inclusive. The Reporting Persons undertake to provide to the Issuer, any security holder of the Issuer, or the staff of the Securities and Exchange Commission, upon request, full information regarding the number of shares sold at each separate price within the ranges set forth in footnotes (2), (4), (5) and (6). ( 3 )The shares are held directly by Atlas Venture Fund XI, L.P. ("Atlas Venture Fund XI"). The general partner of Atlas Venture Fund XI is Atlas Venture Associates XI, L.P. ("AVA XI LP"). Atlas Venture Associates XI, LLC ("AVA XI LLC") is the general partner of AVA XI LP. Each of AVA XI LP and AVA XI LLC disclaims Section 16 beneficial ownership of the securities held by Atlas Venture Fund XI, except to the extent of its pecuniary interest therein, if any. ( 4 )The price reported in Column 4 is a weighted average price. These shares were sold in multiple transactions at prices ranging from $22.7625 to $23.3899 inclusive. ( 5 )The price reported in Column 4 is a weighted average price. These shares were sold in multiple transactions at prices ranging from $22.17 to $23.1566 inclusive. ( 6 )The price reported in Column 4 is a weighted average price. These shares were sold in multiple transactions at prices ranging from $22.205 to $22.92 inclusive. ( 7 )This transaction was made pursuant to a Rule 10b5-1 trading plan adopted by Atlas Venture Opportunity Fund I, L.P. on January 10, 2024. ( 8 )The shares are held directly by Atlas Venture Opportunity Fund I, L.P. ("AVOF I"). The general partner of AVOF I is Atlas Venture Associates Opportunity I, L.P. ("AVAO I LP"). Atlas Venture Associates Opportunity I, LLC ("AVAO I LLC") is the general partner of AVAO I LP. Each of AVAO I LP and AVAO I LLC disclaims Section 16 beneficial ownership of the securities held by AVOF I, except to the extent of its pecuniary interest therein, if any. ( 9 )This transaction was made pursuant to a Rule 10b5-1 trading plan adopted by Atlas Venture Opportunity Fund II, L.P. on January 10, 2024. ( 10 )The shares are owned directly by Atlas Venture Opportunity Fund II, L.P. ("AVOF II"). Atlas Venture Associates Opportunity II, L.P. ("AVAO II LP") is the general partner of AVOF II. Atlas Venture Associates Opportunity II, LLC ("AVAO II LLC") is the general partner of AVAO II LP. Each of AVAO II LP and AVAO II LLC disclaims Section 16 beneficial ownership of the securities held by AVOF II, except to the extent of its pecuniary interest therein, if any.

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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