CUSIP No. 15708Q105

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: April 21, 2014

POLARIS VENTURE PARTNERS V, L.P.
By:	Polaris Venture Management Co. V, L.L.C.
By:	/s/ John Gannon
	Authorized Signatory
POLARIS VENTURE PARTNERS ENTREPRENEURS’ FUND V, L.P.
By:	Polaris Venture Management Co. V, L.L.C.
By:	/s/ John Gannon
	Authorized Signatory
POLARIS VENTURE PARTNERS FOUNDERS’ FUND V, L.P.
By: Polaris Venture Management Co. V, L.L.C.
By:	/s/ John Gannon
	Authorized Signatory

CUSIP No. 15708Q105

POLARIS VENTURE PARTNERS SPECIAL FOUNDERS’ FUND V, L.P.
By:	Polaris Venture Management Co. V, L.L.C.
By:	/s/ John Gannon
	Authorized Signatory
POLARIS VENTURE MANAGEMENT CO. V, L.L.C.
By:	/s/ John Gannon
	Authorized Signatory
JONATHAN A. FLINT
By:	/s/ Jonathan A. Flint
	Jonathan A. Flint
TERRANCE G. MCGUIRE
By:	/s/ Terrance G. McGuire
	Terrance G. McGuire