Exhibit A AGREEMENT REGARDING THE JOINT FILING OF SCHEDULE 13D

CUSIP No. 000000000 Page 10 of 10

Exhibit A

AGREEMENT REGARDING THE JOINT FILING OF

SCHEDULE 13D

The undersigned, being duly authorized thereunto, hereby execute this agreement as an exhibit to this Schedule 13D to evidence the agreement of the below-named parties, in accordance with the rules promulgated pursuant to the Securities Exchange Act of 1934, to file this Schedule jointly on behalf of each such party.

Date: October 20, 2017

LONGWOOD FUND II, L.P.
By:		Longwood Fund II GP, LLC, its general partner
By:		/s/ Xxxxxxxxx Xxxxxxxx, M.D., Ph.D.
		Manager
LONGWOOD FUND II GP, LLC
By:		/s/ Xxxxxxxxx Xxxxxxxx, M.D., Ph.D.
		Manager
XXXXXXXXX XXXXXXXX, M.D., PH.D.
By:		/s/ Xxxxxxxxx Xxxxxxxx, M.D., Ph.D.
		Xxxxxxxxx Xxxxxxxx, M.D., Ph.D.
XXXXXXX XXXXXXX
By:		/s/ Xxxxxxx Xxxxxxx
		Xxxxxxx Xxxxxxx