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

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Ownership Summary Table

Name Sole Voting Power Shared Voting Power Sole Dispositive Power Shared Dispositive Power Aggregate Amount Owned Power Percent of Class
Baker Bros. Advisors 45,834,076 0 45,834,076 0 45,834,076 32.1%
Baker Bros. Advisors (GP) 45,834,076 0 45,834,076 0 45,834,076 32.1%
Julian C. Baker 45,972,501 0 45,972,501 0 45,972,501 32.2%
Felix J. Baker 45,972,508 0 45,972,508 0 45,972,508 32.2%
FBB 18,243 0 18,243 0 18,243 (1)%
FBB3 12,678 0 12,678 0 12,678 (1)%

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Washington, D.C. 20549


Under the Securities Exchange Act of 1934

(Amendment No. 24)

Seattle Genetics, Inc.

(Name of Issuer)

Common Stock, par value $0.001 per share

(Title of Class of Securities)


(CUSIP Number)

Alexandra A. Toohey

Chief Financial Officer

Baker Bros. Advisors LP
667 Madison Avenue, 21st Floor
New York, NY 10065

(212) 339-5690

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

May 15, 2017

(Date of Event which Requires Filing of this

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).

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