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

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Page 2 of 12 – SEC Filing

SCHEDULE 13D

CUSIP No.   812578102       Page   2   of   12   Pages
1

NAMES OF REPORTING PERSONS

Baker Bros. Advisors LP

2 CHECK THE APPROPRIATE BOX IF A MEMBER OF A GROUP (See Instructions)

(a) ¨

(b) ¨

3 SEC USE ONLY
4

SOURCE OF FUNDS (See Instructions)

OO

5

CHECK
BOX IF DISCLOSURE OF LEGAL PROCEEDINGS IS REQUIRED PURSUANT TO ITEMS 2(d) OR 2(e)

¨
6

CITIZENSHIP OR PLACE OF ORGANIZATION

Delaware

NUMBER OF
SHARES
BENEFICIALLY
OWNED BY
EACH
REPORTING
PERSON
WITH
7

SOLE VOTING POWER

45,834,076 (1)

8

SHARED VOTING POWER

0

9

SOLE DISPOSITIVE POWER

45,834,076 (1)

10

SHARED DISPOSITIVE POWER

0

11

AGGREGATE AMOUNT BENEFICIALLY OWNED BY EACH REPORTING PERSON

45,834,076 (1)

12

CHECK
BOX IF THE AGGREGATE AMOUNT IN ROW (11) EXCLUDES CERTAIN SHARES (See Instructions)

¨
13

PERCENT OF CLASS REPRESENTED BY AMOUNT IN ROW (11)

32.1% (1)(2)

14

TYPE OF REPORTING PERSON (See Instructions)

IA, PN

(1) Includes 108,520 shares of the Issuer’s common stock underlying
108,520 options.

(2) Based
on 142,716,597 shares of common stock
as of April 26, 2017 as reported in the Issuer’s 10-Q filed with the SEC on
May 1, 2017.

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