Exhibit A AGREEMENT REGARDING THE JOINT FILING OF SCHEDULE 13D

CUSIP No. 000000000 Page 13 of 14

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: February 16, 2016

POLARIS VENTURE PARTNERS V, L.P.
By: Polaris Venture Management Co. V, L.L.C.
By:		*
		Managing Member
POLARIS VENTURE PARTNERS ENTREPRENEURS’ FUND V, L.P.
By: Polaris Venture Management Co. V, L.L.C.
By:		*
		Managing Member
POLARIS VENTURE PARTNERS FOUNDERS’ FUND V, L.P.
By: Polaris Venture Management Co. V, L.L.C.
By:		*
		Managing Member

CUSIP No. 000000000 Page 14 of 14

POLARIS VENTURE PARTNERS SPECIAL FOUNDERS’ FUND V, L.P.
By: Polaris Venture Management Co. V, L.L.C.
By:		*
		Managing Member
POLARIS VENTURE MANAGEMENT CO. V, L.L.C.
By:		*
		Managing Member
XXXXXXXX X. XXXXX
By:		*
		Xxxxxxxx X. Xxxxx
XXXXXXXX X. XXXXXXX
By:		*
		Xxxxxxxx X. XxXxxxx
XXXXX XXXXXXXXX
By:		*
		Xxxxx Xxxxxxxxx

*By:		/s/ Xxxx Xxxxx
Name:		Xxxx Xxxxx
		Attorney-in-Fact

[This Schedule 13D was executed pursuant to a Power of Attorney. Note that copies of the applicable Powers of Attorney are already on file with the appropriate agencies.]