X0306 4 2021-09-16 0 0001690585 Magenta Therapeutics, Inc. MGTA 0001741389 Mahoney Stephen F. C/O MAGENTA THERAPEUTICS, INC. 100 TECHNOLOGY SQUARE CAMBRIDGE MA 02139 0 1 0 0 See Remarks Common Stock 2021-09-16 4 S 0 5736 6.73 D 14264 D This transaction represents shares sold solely to pay taxes due upon the vesting of restricted stock units, as disclosed by the reporting person on the Form 4 filed on September 16, 2021. The price reported in column 4 is a weighted average price. These shares were sold in multiple transactions at prices ranging from $6.60 to $6.87, inclusive. The reporting person undertakes to provide 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. Chief Financial and Operating Officer /s/ Cindy Driscoll, Attorney-in-Fact for Stephen F. Mahoney 2021-09-20