EXHIBIT 99.1

JOINT FILING AGREEMENT

In accordance with Rule 13d-1(k)(1) under the Securities Exchange Act of 1934, as amended, the undersigned hereby agree that this Statement on Schedule 13D relating to the Common Stock, $0.001 par value, of Seattle Genetics, Inc. is being filed with the Securities and Exchange Commission on behalf of each of them.

December 16, 2013

BAKER BROS. ADVISORS LP
By:  Baker Bros. Advisors (GP) LLC, its general partner
By:	/s/ Scott L. Lessing
	Name: Scott L. Lessing Title: President

BAKER BROS. ADVISORS (GP) 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

FBB2, LLC
By:	/s/ Julian C. Baker
	Name: Julian C. Baker
	Title: Manager