X0306 4 2019-02-19 0 0001750019 TCR2 THERAPEUTICS INC. TCRR 0001609492 MPM BioVentures 2014, L.P. C/O MPM CAPITAL 450 KENDALL STREET CAMBRIDGE MA 02142 0 0 1 0 0001473930 Foley Todd C/O MPM CAPITAL 450 KENDALL STREET CAMBRIDGE MA 02142 0 0 1 0 0001134657 EVNIN LUKE C/O MPM CAPITAL 450 KENDALL STREET CAMBRIDGE MA 02142 0 0 1 0 0001609493 MPM BioVentures 2014 (B), L.P. C/O MPM CAPITAL 450 KENDALL STREET CAMBRIDGE MA 02142 0 0 1 0 0001609495 MPM Asset Management Investors BV2014 LLC C/O MPM CAPITAL 450 KENDALL STREET CAMBRIDGE MA 02142 0 0 1 0 0001765021 MPM BioVentures 2014 LLC C/O MPM CAPITAL 450 KENDALL STREET CAMBRIDGE MA 02142 0 0 1 0 0001765091 MPM BioVentures 2014 GP LLC C/O MPM CAPITAL 450 KENDALL STREET CAMBRIDGE MA 02142 0 0 1 0 0001263048 MPM ASSET MANAGEMENT LLC C/O MPM CAPITAL 450 KENDALL STREET CAMBRIDGE MA 02142 0 0 1 0 Common Stock 2019-02-19 4 C 0 2825396 A 3021298 I See Footnote Common Stock 2019-02-19 4 C 0 322901 A 3344199 I See Footnote Common Stock 2019-02-19 4 P 0 706666 15.00 A 4050865 I See Footnote Series A Preferred Stock 2019-02-19 4 C 0 17500000 0.00 D Common Stock 2825396 0 I See Footnote Series B Preferred Stock 2019-02-19 4 C 0 2000000 0.00 D Common Stock 322901 0 I See Footnote All shares of Series A Preferred Stock and Series B Preferred Stock automatically converted into the number of shares of the Issuer's common stock on a 6.1938:1 basis, for no additional consideration, immediately prior to the closing of the Issuer's initial public offering and had no expiration date. The Reporting Persons disclaim beneficial ownership of the securities except to the extent of their respective pecuniary interests therein. The shares are held as follows: 2,195,681 by MPM BioVentures 2014, L.P. ("BV 2014"), 146,447 by MPM BioVentures 2014(B), L.P. ("BV 2014(B)"), 79,644 by MPM Asset Management Investors BV2014 LLC ("AM BV2014"), 195,902 by MPM Asset Management LLC ("AM LLC"), 351,155 by MPM SunStates Fund, L.P. ("MPM SunStates") and 52,469 by MPM Asset Management Investors Sunstates Fund LLC ("AM SunStates"). MPM BioVentures 2014 GP LLC and MPM BioVentures 2014 LLC ("BV LLC") are the direct and indirect general partners of BV 2014 and BV 2014(B). MPM SunStates Fund GP LLC and MPM SunStates GP Managing Member LLC ("SunStates GP") are the direct and indirect general partners of MPM SunStates. Luke Evnin, Ansbert Gadicke and Todd Foley are the managing directors of BV LLC, Ansbert Gadicke is a member of SunStates GP and Luke Evnin and Anbsert Gadicke are the members of AM LLC. The shares are held as follows: 2,451,844 by BV 2014, 163,532 by BV 2014(B), 88,935 by AM BV2014, 195,902 by AM LLC, 386,270 by MPM SunStates and 57,716 by AM SunStates. The shares are held as follows: 3,056,272 by BV 2014, 203,846 by BV 2014(B), 110,859 by AM BV2014, 195,902 by AM LLC, 421,070 by MPM SunStates and 62,916 by AM SunStates. No securities held by the Reporting Persons. See Form 4 for MPM SunStates Fund, L.P for additional members of this joint filing. /s/ Ansbert Gadicke, managing director of MPM BioVentures 2014 LLC, the managing member of MPM BioVentures 2014 GP LLC, the general partner of MPM BioVentures 2014, L.P. 2019-02-21 /s/ Todd Foley 2019-02-21 /s/ Luke Evnin 2019-02-21 /s/ Ansbert Gadicke, managing director of MPM BioVentures 2014 LLC, the managing member of MPM BioVentures 2014 GP LLC, the general partner of MPM BioVentures 2014 (B), L.P. 2019-02-21 /s/ Howard Rubin, director of MPM Asset Management Investors BV2014 LLC 2019-02-21 /s/ Ansbert Gadicke, managing director of MPM BioVentures 2014 LLC 2019-02-21 /s/ Ansbert Gadicke, managing director of MPM BioVentures 2014 LLC, the managing member of MPM BioVentures 2014 GP LLC 2019-02-21 /s/ Ansbert Gadicke, member of MPM Asset Management LLC 2019-02-21