Exhibit A

JOINT FILING AGREEMENT

The undersigned agree that this Schedule 13D/A dated November 24, 2008, relating to the Common Stock of CRXX shall be filed on behalf of the undersigned.

Dated:	November 24, 2008
	BIOTECHNOLOGY VALUE FUND, L.P.
	By:	BVF Partners L.P., its general partner
		By:		BVF Inc., its general partner
				By:	/s/ Mark N. Lampert
					_________________________________
					Mark N. Lampert President
	BIOTECHNOLOGY VALUE FUND II, L.P.
	By:	BVF Partners L.P., its general partner
		By:		BVF Inc., its general partner
				By:	/s/ Mark N. Lampert
					_________________________________
					Mark N. Lampert President
	BVF INVESTMENTS, L.L.C.
	By:	BVF Partners L.P., its manager
		By:		BVF Inc., its general partner
				By:	/s/ Mark N. Lampert
					_________________________________
					Mark N. Lampert President
	INVESTMENT 10, L.L.C.
	By:	BVF Partners L.P., its investment manager
		By:	BVF Inc., its general partner
				By:	/s/ Mark N. Lampert
					_________________________________
					Mark N. Lampert President
	BVF PARTNERS L.P.
	By:	BVF Inc., its general partner
		By:		/s/ Mark N. Lampert
				_________________________________
				Mark N. Lampert President
	BVF INC.
	By:	/s/ Mark N. Lampert
		_________________________________
		Mark N. Lampert President