EXHIBIT 4

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.

April 24, 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