EXHIBIT 99.1

Joint Filing Agreement

The undersigned acknowledge and agree that the foregoing statement on Schedule 13G is filed on behalf of each of the undersigned and that all subsequent amendments to this statement on Schedule 13G shall be filed on behalf of each of the undersigned without the necessity of filing additional joint filing statements. The undersigned acknowledge that each shall be responsible for the timely filing of such amendments, and for the completeness and accuracy of the information concerning him, her or it contained herein, but shall not be responsible for the completeness and accuracy of the information concerning the other entities or persons, except to the extent that he, she or it knows or has reason to believe that such information is inaccurate.

Dated: February 13, 2017

THIRD ROCK VENTURES, L.P.
By:	THIRD ROCK VENTURES GP, L.P.,
	General Partner
By:	TRV GP, LLC,
	General Partner
By:	/s/ Kevin Gillis
	Kevin Gillis
	Chief Financial Officer
THIRD ROCK VENTURES GP, L.P.
By:	TRV GP, LLC,
	General Partner
By:	/s/ Kevin Gillis
	Kevin Gillis
	Chief Financial Officer
TRV GP, LLC
By:	/s/ Kevin Gillis
	Kevin Gillis
	Chief Financial Officer
MARK LEVIN
/s/ Kevin Gillis, As attorney-in-fact
Mark Levin

KEVIN P. STARR

/s/ Kevin Gillis, As attorney-in-fact

Kevin P. Starr

ROBERT I. TEPPER

/s/ Kevin Gillis, As attorney-in-fact

Robert I. Tepper