EXHIBIT A

JOINT FILING AGREEMENT

In accordance with Rule 13d-1(k) under the Securities Exchange Act of 1934, as amended, the undersigned agree to the joint filing on behalf of each of them of a statement on Schedule 13G (including amendments thereto) with respect to the Common Stock of Dyne Therapeutics, Inc. and further agree that this agreement be included as an exhibit to such filing. Each party to the agreement expressly authorizes each other party to file on its behalf any and all amendments to such statement. Each party to this agreement agrees that this joint filing agreement may be signed in counterparts.

In evidence whereof, the undersigned have caused this Agreement to be executed on their behalf this 11th day of February, 2021.

MPM BIOVENTURES 2018, L.P.
By:	MPM BioVentures 2018 GP LLC,
	its General Partner
By:	MPM BioVentures 2018 LLC,
	Its Managing Member
By:	/s/ Luke Evnin
Name:	Luke Evnin
Title:	Managing Director
MPM BIOVENTURES 2018 (B), L.P.
By:	MPM BioVentures 2018 GP LLC,
	its General Partner
By:	MPM BioVentures 2018 LLC,
	Its Managing Member
By:	/s/ Luke Evnin
Name:	Luke Evnin
Title:	Managing Director
MPM ASSET MANAGEMENT INVESTORS BV2018 LLC
By:	MPM BioVentures 2018 LLC,
	Its Manager
By:	/s/ Luke Evnin
Name:	Luke Evnin
	Title: Managing Director
MPM BIOVENTURES 2018 GP LLC
By:	MPM BioVentures 2018 LLC,
	Its Managing Member
By:	/s/ Luke Evnin
Name:	Luke Evnin
Title:	Managing Director

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MPM BIOVENTURES 2018 LLC
By:	/s/ Luke Evnin
Name:	Luke Evnin
Title:	Managing Director
By:	/s/ Todd Foley
Name:	Todd Foley
By:	/s/ Luke Evnin
Name:	Luke Evnin
By:	/s/ Ansbert Gadicke
Name:	Ansbert Gadicke
By:	/s/ Edward Hurwitz
Name:	Edward Hurwitz

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