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 Syndax Pharmaceuticals, 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 31^st day of January, 2018.

MPM BIOVENTURES IV-QP, L.P.
By:	MPM BioVentures IV GP LLC,
	its General Partner
By:	MPM BioVentures IV LLC,
	its Managing Member
By:	/s/ Luke Evnin
	Name: Luke Evnin
	Title: Member
MPM ASSET MANAGEMENT INVESTORS BV4 LLC
By:	MPM BioVentures IV LLC
	its Manager
By:	/s/ Luke Evnin
	Name: Luke Evnin
	Title: Member
MPM BIOVENTURES IV GP LLC
By:	MPM BioVentures IV LLC,
	its Managing Member
By:	/s/ Luke Evnin
Name:	Luke Evnin
Title:	Member
By:	/s/ Luke Evnin
	Name: Luke Evnin
By:	/s/ Todd Foley
	Name: Todd Foley

MPM BIOVENTURES IV GMBH & CO. BETEILIGUNGS KG
By:	MPM BioVentures IV GP LLC,
	in its capacity as the Managing Limited Partner
By:	MPM BioVentures IV LLC,
	its Managing Member
By:	/s/ Luke Evnin
	Name: Luke Evnin
	Title: Member
MPM BIOVENTURES IV STRATEGIC FUND, L.P.
By:	MPM BioVentures IV GP LLC,
	its General Partner
By:	MPM BioVentures IV LLC,
	its Managing Member
By:	/s/ Luke Evnin
	Name: Luke Evnin
	Title: Member
MPM BIOVENTURES IV LLC
By:	/s/ Luke Evnin
Name:	Luke Evnin
Title:	Member
By:	/s/ Ansbert Gadicke
	Name: Ansbert Gadicke