Sec Form 4 Filing - Atlas Venture Fund X, L.P. @ Magenta Therapeutics, Inc. - 2019-03-20

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 X, 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 X __ 10% Owner _____ Officer (give title below) _____ Other (specify below) (Last) (First) (Middle) 400 TECHNOLOGY SQUARE, 10TH FLOOR 3. Date of Earliest Transaction (MM/DD/YY) 03/20/2019 (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 03/20/2019 J( 1 ) 726,500 D $ 0 4,117,125 D ( 2 ) Common Stock 03/20/2019 J( 3 ) 14,245 A $ 0 14,245 I See Footnote ( 4 ) Common Stock 03/20/2019 S 14,245 D $ 17.86 ( 5 ) 0 I See Footnote ( 4 )

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 X, L.P. 400 TECHNOLOGY SQUARE, 10TH FLOOR CAMBRIDGE, MA02139 X ATLAS VENTURE ASSOCIATES X, L.P. 400 TECHNOLOGY SQUARE, 10TH FLOOR CAMBRIDGE, MA02139 X Atlas Venture Associates X, LLC 400 TECHNOLOGY SQUARE, 10TH FLOOR CAMBRIDGE, MA02139 X Signatures Atlas Venture Fund X, L.P., By: Atlas Venture Associates X, L.P., its general partner, by Atlas Associates X, LLC, its general partner, By: Ommer Chohan, Chief Financial Officer /s/ Ommer Chohan 03/22/2019 Signature of Reporting Person Date Atlas Venture Associates X, LP, By: Atlas Venture Associates X, LLC, its general partner, By: Ommer Chohan, Chief Financial Officer /s/ Ommer Chohan 03/22/2019 Signature of Reporting Person Date Atlas Venture Associates X, LLC, By: Ommer Chohan, Chief Financial Officer /s/ Ommer Chohan 03/22/2019 Signature of Reporting Person Date

Explanation of Responses:

( 1 )Represents a pro rata distribution, and not a purchase or sale of securities, by Atlas Venture Fund X, L.P. ("AVF X") to its general partner and limited partners without additional consideration. ( 2 )The shares are held directly by AVF X. The general partner of AVF X is Atlas Venture Associates X, L.P. ("AVA X LP"). Atlas Venture Associates X, LLC ("AVA X LLC") is the general partner of AVA X LP. Each of AVA X LP and AVA X LLC disclaims Section 16 beneficial ownership of the securities held by AVF X, except to the extent of its pecuniary interest therein, if any. ( 3 )Represents the shares received by AVA X LP in connection with the pro rata distribution described in footnote 1 by AVF X. ( 4 )The shares are held directly by AVA X LP. AVA X LLC is the general partner of AVA X LP. AVA X LLC disclaims Section 16 beneficial ownership of the securities held by AVA X LP, except to the extent of its pecuniary interest therein, if any. ( 5 )The price reported in Column 4 is a weighted average price. These shares were sold by AVA X LP in multiple transactions at prices ranging from $17.75 to $18.14, inclusive. The Reporting Person undertakes 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 range set forth in this footnote.

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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