X0206 3 2020-09-16 0 0001818794 Dyne Therapeutics, Inc. DYN 0001729505 MPM BIOVENTURES 2018, L.P. 450 KENDALL STREET CAMBRIDGE MA 02142 0 0 1 0 0001729504 MPM BIOVENTURES 2018 (B), L.P. 450 KENDALL STREET CAMBRIDGE MA 02142 0 0 1 0 0001734817 MPM ASSET MANAGEMENT INVESTORS BV2018 LLC 450 KENDALL STREET CAMBRIDGE MA 02142 0 0 1 0 0001818701 MPM BioVentures 2018 GP LLC 450 KENDALL STREET CAMBRIDGE MA 02142 0 0 1 0 0001818702 MPM BioVentures 2018 LLC 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 0001134655 GADICKE ANSBERT C/O MPM CAPITAL 450 KENDALL STREET CAMBRIDGE MA 02142 0 0 1 0 Series A Preferred Stock Common Stock 4215076 I See footnote Series A Preferred Stock Common Stock 224025 I See footnote Series A Preferred Stock Common Stock 83188 I See footnote Series B Preferred Stock Common Stock 399867 I See footnote Series B Preferred Stock Common Stock 21254 I See footnote Series B Preferred Stock Common Stock 7893 I See footnote The Series A Preferred Stock is convertible into common stock on a 3.3169-for-1 basis into the number of shares of common stock shown in Column 3 without payment of further consideration at the holder's election or upon closing of the initial public offering of the Issuer's common stock. The shares have no expiration date. The shares are held directly by MPM BioVentures 2018, L.P. ("BV 2018"). The general partner of BV 2018 is MPM BioVentures 2018 GP LLC ("BV 2018 GP"). MPM BioVentures 2018 LLC ("BV 2018 LLC") is the managing member of BV 2018 GP. Edward Hurwitz, Luke Evnin, Ansbert Gadicke, and Todd Foley are the Managing Directors of BV 2018 LLC. Each of BV 2018 GP, BV 2018 LLC, Edward Hurwitz, Luke Evnin, Ansbert Gadicke, and Todd Foley disclaims Section 16 beneficial ownership of the securities held by BV 2018, except to the extent of its pecuniary interest therein, if any. The shares are held directly by MPM BioVentures 2018 (B), L.P. ("BV 2018(B)"). The general partner of BV 2018(B) is BV 2018 GP. BV 2018 LLC is the managing member of BV 2018 GP. Edward Hurwitz, Luke Evnin, Ansbert Gadicke, and Todd Foley are the Managing Directors of BV 2018 LLC. Each of BV 2018 GP, BV 2018 LLC, Edward Hurwitz, Luke Evnin, Ansbert Gadicke, and Todd Foley disclaims Section 16 beneficial ownership of the securities held by BV 2018(B), except to the extent of its pecuniary interest therein, if any. The shares are held directly by MPM Asset Management Investors BV2018 LLC ("MPM Asset Management"). The manager of MPM Asset Management is BV 2018 LLC. Edward Hurwitz, Luke Evnin, Ansbert Gadicke, and Todd Foley are the Managing Directors of BV 2018 LLC. Each of BV 2018 LLC, Edward Hurwitz, Luke Evnin, Ansbert Gadicke, and Todd Foley disclaims Section 16 beneficial ownership of the securities held by MPM Asset Management, except to the extent of his pecuniary interest therein, if any. The Series B Preferred Stock is convertible into common stock on a 3.3169-for-1 basis into the number of shares of common stock shown in Column 3 without payment of further consideration at the holder's election or upon closing of the initial public offering of the Issuer's common stock. The shares have no expiration date. /s/ Ed Hurwitz, managing director of MPM BioVentures 2018 LLC, the managing member of MPM BioVentures 2018 GP LLC, the general partner of MPM BioVentures 2018, L.P. 2020-09-16 /s/ Ed Hurwitz, managing director of MPM BioVentures 2018 LLC, the managing member of MPM BioVentures 2018 GP LLC, the general partner of MPM BioVentures 2018 (B), L.P. 2020-09-16 /s/ Ed Hurwitz, managing director of MPM BioVentures 2018 LLC, the manager of MPM Asset Management Investors BV2018 LLC 2020-09-16 /s/ Ed Hurwitz, managing director of MPM BioVentures 2018 LLC, the managing member of MPM BioVentures 2018 GP LLC 2020-09-16 /s/ Ed Hurwitz, managing director of MPM BioVentures 2018 LLC 2020-09-16 /s/ Luke Evnin 2020-09-16 /s/ Todd Foley 2020-09-16 /s/ Ansbert Gadicke 2020-09-16