Exhibit 1

JOINT FILING AGREEMENT

Pursuant to Rule 13d-1(k)(1) under the Securities Exchange Act of 1934, as amended, the undersigned hereby agree that only one statement containing the information required by Schedule 13D need be filed with respect to the ownership by each of the undersigned of the shares of Common Stock of Xoma Corporation.

This Agreement may be executed in any number of counterparts, each of which shall be deemed an original.

EXECUTED as of this 20th day of April, 2012.

	BAKER BROS. ADVISORS, LLC

	By:	/s/ Scott L. Lessing
		Name: Scott L. Lessing Title: President

		/s/ Julian C. Baker
		Julian C. Baker

		/s/ Felix J. Baker
		Felix J. Baker