Atlas Venture Fund X SC 13D/A Magenta Therapeutics / Atlas Venture Fund X ownership change

MAGENTA THERAPEUTICS, INC. SC 13D/A

EXHIBIT B

JOINT FILING AGREEMENT

Pursuant to Rule 13d-1(k)(1) promulgated pursuant to the Securities Exchange Act of 1934, as amended, the undersigned agree that the attached Schedule 13D is being filed on behalf of each of the undersigned.

Dated: February 17, 2021

ATLAS VENTURE FUND X, L.P.
By: Atlas Venture Associates X, L.P., its general partner
By: Atlas Venture Associates X, LLC, its general partner
By:	/s/ Ommer Chohan
Name: Ommer Chohan
Title: Chief Financial Officer
ATLAS VENTURE ASSOCIATES X, L.P.
By: Atlas Venture Associates X, LLC, its general partner

By:	/s/ Ommer Chohan
Name: Ommer Chohan
Title: Chief Financial Officer
ATLAS VENTURE ASSOCIATES X, LLC
By:	/s/ Ommer Chohan
Name: Ommer Chohan
Title: Chief Financial Officer
ATLAS VENTURE OPPORTUNITY FUND I, L.P.
By: Atlas Venture Associates Opportunity I, L.P., its general partner
By: Atlas Venture Associates Opportunity I, LLC, its general partner
By:	/s/ Ommer Chohan
Name: Ommer Chohan
Title: Chief Financial Officer
ATLAS VENTURE ASSOCIATES OPPORTUNITY I, L.P.
By: Atlas Venture Associates Opportunity I, LLC, its general partner
By:	/s/ Ommer Chohan
Name: Ommer Chohan
Title: Chief Financial Officer
ATLAS VENTURE ASSOCIATES OPPORTUNITY I, LLC
By:	/s/ Ommer Chohan
Name: Ommer Chohan
Title: Chief Financial Officer