X0306 4 2020-07-31 0 0001818794 Dyne Therapeutics, Inc. DYN 0001211840 HURWITZ EDWARD C/O DYNE THERAPEUTICS, INC. 830 WINTER STREET WALTHAM MA 02451 1 0 1 0 Common Stock 2020-09-21 4 C 0 4614943 A 4614943 I See footnote Common Stock 2020-09-21 4 C 0 245279 A 245279 I See footnote Common Stock 2020-09-21 4 C 0 91081 A 91081 I See footnote Series A Preferred Stock 2020-07-31 4 P 0 4893347 1.00 A Common Stock 1475277 13980992 I See footnote Series A Preferred Stock 2020-07-31 4 P 0 260077 1.00 A Common Stock 78409 743076 I See footnote Series A Preferred Stock 2020-07-31 4 P 0 96576 1.00 A Common Stock 29116 275932 I See footnote Series B Preferred Stock 2020-08-07 4 P 0 1326313 2.811 A Common Stock 399867 1326313 I See footnote Series B Preferred Stock 2020-08-07 4 P 0 70492 2.811 A Common Stock 21254 70492 I See footnote Series B Preferred Stock 2020-08-07 4 P 0 26176 2.811 A Common Stock 7893 26176 I See footnote Series A Preferred Stock 2020-09-21 4 C 0 13980992 0.00 D Common Stock 4215076 0 I See footnote Series A Preferred Stock 2020-09-21 4 C 0 743076 0.00 D Common Stock 224025 0 I See footnote Series A Preferred Stock 2020-09-21 4 C 0 275932 0.00 D Common Stock 83188 0 I See footnote Series B Preferred Stock 2020-09-21 4 C 0 1326313 0.00 D Common Stock 399867 0 I See footnote Series B Preferred Stock 2020-09-21 4 C 0 70492 0.00 D Common Stock 21254 0 I See footnote Series B Preferred Stock 2020-09-21 4 C 0 26176 0.00 D Common Stock 7893 0 I See footnote On September 21, 2020, the Series A Preferred Stock automatically converted into Common Stock on a 3.3169-for-one basis without payment of further consideration upon closing of the initial public offering of the Issuer's common stock. The shares had no expiration date. On September 21, 2020, the Series B Preferred Stock automatically converted into Common Stock on a 3.3169-for-one basis without payment of further consideration upon closing of the initial public offering of the Issuer's common stock. The shares had 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. The Reporting Person, Luke Evnin, Ansbert Gadicke, and Todd Foley are the Managing Directors of BV 2018 LLC. The Reporting Person disclaims Section 16 beneficial ownership of the securities held by BV 2018, except to the extent of his 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. The Reporting Person, Luke Evnin, Ansbert Gadicke, and Todd Foley are the Managing Directors of BV 2018 LLC. The Reporting Person disclaims Section 16 beneficial ownership of the securities held by BV 2018(B), except to the extent of his 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. The Reporting Person, Luke Evnin, Ansbert Gadicke, and Todd Foley are the Managing Directors of BV 2018 LLC. The Reporting Person 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 A Preferred Stock was convertible into common stock on a 3.3169-for-one basis into the number of shares of common stock shown in Column 7 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 had no expiration date. The Series B Preferred Stock was convertible into common stock on a 3.3169-for-one basis into the number of shares of common stock shown in Column 7 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 had no expiration date. /s/ Richard Scalzo, Attorney-in-Fact 2020-09-23