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

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Page 3 of 12 – SEC Filing

CUSIP No.   812578102       Page   3   of   12   Pages
1

NAMES OF REPORTING PERSONS

Baker Bros. Advisors (GP) LLC

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

49,680,229 (1)

8

SHARED VOTING POWER

0

9

SOLE DISPOSITIVE POWER

49,680,229 (1)

10

SHARED DISPOSITIVE POWER

0

11

AGGREGATE AMOUNT BENEFICIALLY OWNED BY EACH REPORTING PERSON

49,680,229 (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)

31.6% (1)(2)

14

TYPE OF REPORTING PERSON (See Instructions)

HC, OO

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

(2) Based
on 155,341,860 shares of the Issuer’s Common Stock that will be outstanding following the Offering (as defined below) plus
1,730,769 shares issued as part of the Underwriter’s option (as defined below) described in the Issuer’s Prospectus
Supplement filed with the SEC on February 1, 2018.

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