13D Filing: Baker Bros. Advisors and Seattle Genetics Inc (SGEN)

UNITED STATES
SECURITIES AND EXCHANGE COMMISSION
Washington, D.C. 20549

SCHEDULE 13D
Under the Securities Exchange Act of 1934

(Amendment No. 25)

Seattle
Genetics, Inc.

(Name of Issuer)

Common Stock, par value $0.001 per share

(Title of Class of Securities)

812578102

(CUSIP Number)

Alexandra A. Toohey

Chief Financial Officer

Baker Bros. Advisors LP
860 Washington Street, 3^rd Floor
New York, NY 10014
(212) 339-5690

(Name, Address and Telephone Number of Person
Authorized to Receive Notices and Communications)

January 31, 2018

(Date of Event which Requires Filing of
this Statement)

If the filing person has previously filed
a statement on Schedule 13G to report the acquisition that is the subject of this Schedule 13D, and is filing this schedule because
of Rule 13d-1(e), 13d-1(f) or 13d-1(g), check the following box. ¨

 

Note: Schedules filed in paper format
shall include a signed original and five copies of the schedule, including all exhibits. See Rule 13d-7 for other parties to whom
copies are to be sent.

*The remainder of this cover page shall
be filled out for a reporting person’s initial filing on this form with respect to the subject class of securities, and for
any subsequent amendment containing information which would alter disclosures provided in a prior cover page.

The information required on the remainder
of this cover page shall not be deemed to be “filed” for the purpose of Section 18 of the Securities Exchange Act of
1934 (the “Act”) or otherwise subject to the liabilities of that section of the Act but shall be subject to all other
provisions of the Act (however, see the Notes).