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EXHIBIT 99A

EXHIBIT A
AGREEMENT REGARDING JOINT FILING

    The undersigned, Biotechnology Value Fund, L.P., a Delaware limited partnership, Biotechnology Value Fund II, L.P., a Delaware limited partnership, BVF Investments, L.L.C., a Delaware limited liability company, BVF Partners L.P., a Delaware limited partnership, and BVF Inc., a Delaware corporation, hereby agree and acknowledge that the information required by Schedule 13D, to which this Agreement is attached as an exhibit, is filed on behalf of each of them. The undersigned further agree that any amendments or supplements thereto shall also be filed on behalf of each of them.

Dated: December 10, 2001
	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
	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

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EXHIBIT 99A
EXHIBIT A AGREEMENT REGARDING JOINT FILING