Sec Form 3 Filing - Atlas Venture Opportunity Fund I, L.P. @ Magenta Therapeutics, Inc. - 2020-06-29

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 Atlas Venture Opportunity Fund I, L.P. 2. Issuer Name and Ticker or Trading Symbol Magenta Therapeutics, Inc. [ MGTA] 5. Relationship of Reporting Person(s) to Issuer (Check all applicable) _____ Director _____ 10% Owner _____ Officer (give title below) X __ Other (specify below) See remarks (Last) (First) (Middle) 400 TECHNOLOGY SQUARE, 10TH FLOOR 3. Date of Earliest Transaction (MM/DD/YY) 06/29/2020 (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 1,250,000 D ( 1 )

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 Opportunity Fund I, L.P. 400 TECHNOLOGY SQUARE 10TH FLOOR CAMB RIDGE, MA02139 See remarks Atlas Venture Associates Opportunity I, L.P. 400 TECHNOLOGY SQUARE 10TH FLOOR CAMBRIDGE, MA02139 See remarks Atlas Venture Associates Opportunity I, LLC 400 TECHNOLOGY SQUARE 10TH FLOOR CAMBRIDGE, MA See remarks Signatures 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: Ommer Chohan, Chief Financial Officer, /s/ Ommer Chohan 07/13/2020 Signature of Reporting Person Date Atlas Venture Associates Opportunity I, L.P., By: Atlas Venture Associates Opportunity I, LLC, its general partner, By: Ommer Chohan, Chief Financial Officer, /s/ Ommer Chohan 07/13/2020 Signature of Reporting Person Date Atlas Venture Associates Opportunity I, LLC, By: Ommer Chohan, Chief Financial Officer, /s/ Ommer Chohan 07/13/2020 Signature of Reporting Person Date

Explanation of Responses:

( 1 )The shares are held directly by Atlas Venture Opportunity Fund I, L.P. ("Atlas Venture Opportunity Fund I"). The general partner of Atlas Venture Opportunity Fund 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 Atlas Venture Opportunity Fund I, except to the extent of its pecuniary interest therein, if any.

Remarks: The reporting persons purchased the common stock reported herein in an underwritten public offering on June 29, 2020. The reporting persons are under common control with Atlas Venture Fund X, L.P., which, together with the reporting persons, collectively beneficially own more than 10% of the Issuer's outstanding common stock, and as a result the reporting persons and Atlas Venture Fund X, L.P. may be deemed to be members of a group. However, the reporting persons disclaim such group membership, and this report shall not be deemed an admission that the reporting persons are members of a group that owns more than 10% of the outstanding common stock for purposes of Section 16 or for any other purpose.

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).

** Intentional misstatements or omissions of facts constitute Federal Criminal Violations. See 18 U.S.C. 1001 and 15 U.S.C. 78ff(a).

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