X0206 3 2019-02-13 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 0001134657 EVNIN LUKE 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 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 195902 I See footnote Series A Preferred Stock 0.00 Common Stock 2825396 I See footnote Series B Preferred Stock 0.00 Common Stock 322901 I See footnote Common Stock Warrant (Right to Buy) 0.74 2016-12-13 2026-12-12 Common Stock 42761 I See footnote Common Stock Warrant (Right to Buy) 0.74 2017-12-06 2027-12-05 Common Stock 135508 I See footnote The shares are held by MPM Asset Management LLC ("AM LLC"). Luke Evnin and Anbsert Gadicke are the members of AM LLC. Each Reporting Person disclaims beneficial ownership of the securities except to the extent of his or its pecuniary interest therein. Each share of Series A Preferred Stock and Series B Preferred Stock is convertible into shares of the Issuer's Common Stock on a 6.1938:1 basis. Upon the closing of the Issuer's initial public offering, all shares of Series A Preferred Stock and Series B Preferred Stock will convert into shares of Common Stock of the Issuer. The Series A Preferred Stock and Series B Preferred Stock have no expiration dates. 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"), 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 members of BV LLC and Ansbert Gadicke is a member of SunStates GP. The shares are held as follows: 256,163 by BV 2014, 17,085 by BV 2014(B), 9,291 by AM BV2014, 35,115 by MPM SunStates and 5,247 by AM SunStates. The warrants are held by AM LLC. See Form 3 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. 2019-02-13 /s/ Luke Evnin 2019-02-13 /s/ Todd Foley 2019-02-13 /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. 2019-02-13 /s/ Howard Rubin, director of MPM Asset Management Investors BV2014 LLC 2019-02-13 /s/ Ansbert Gadicke, managing director of MPM BioVentures 2014 LLC 2019-02-13 /s/ Ansbert Gadicke, managing director of MPM BioVentures 2014 LLC, the managing member of MPM BioVentures 2014 GP LLC 2019-02-13 /s/ Ansbert Gadicke, member of MPM Asset Management LLC 2019-02-13