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

Amendment No. 25 to Schedule 13D

This Amendment No. 25 to Schedule 13D amends and supplements
the statements on the previously filed Schedule 13D, as amended, filed by Baker Bros. Advisors LP (the “Adviser”),
Baker Bros. Advisors (GP) LLC (the “Adviser GP”), Julian C. Baker, Felix J. Baker, FBB2, LLC (“FBB2”) and
FBB3 LLC (“FBB3”) (collectively the “Reporting Persons”). Except as supplemented herein, such statements,
as heretofore amended and supplemented, remain in full force and effect. Information given in response to each item shall be deemed
incorporated by reference in all other items, as applicable. Each capitalized term used but not defined herein has the meaning
ascribed to such term in the Schedule 13D, as amended.

The Adviser GP is the sole general partner of the Adviser. Pursuant
to management agreements, as amended, among the Adviser, Baker Brothers Life Sciences, L.P. (“Life Sciences”) and 667,
L.P. (“667”, and together with Life Sciences, the “Funds”), and their respective general partners, the
Funds’ respective general partners relinquished to the Adviser all discretion and authority with respect to the investment
and voting power of the securities held by the Funds, and thus the Adviser has complete and unlimited discretion and authority
with respect to the Funds’ investments and voting power over investments.

 

ITEM 2.

Identity and Background.

 

(a) The Reporting Persons are:

1.

The Adviser

2.

The Adviser GP

3.

Felix J. Baker

4.

Julian C. Baker

5.

FBB2

6.

FBB3

(b) The business address of each of the Reporting Persons is:

c/o Baker Bros. Advisors LP

860 Washington Street, 3^rd Floor

New York, NY 10014

(c) The Adviser is an entity engaged in investment activities,
and the Adviser GP is in the business of acting as its general partner and, through the Adviser, investment activities. The principal
business of each of Julian C. Baker and Felix J. Baker is to serve as a managing member of the Adviser GP. The principal business
of FBB2 is to engage in investment activities.  Julian C. Baker and Felix J. Baker are the sole managers of FBB2 and by policy
they do not transact in or vote the securities of Seattle Genetics, Inc. (the “Issuer”) held by FBB2. The principal
business of FBB3 is to engage in investment activities.  Julian C. Baker and Felix J. Baker are the sole managers of FBB3
and by policy they do not transact in or vote the securities of the Issuer held by FBB3.

(d) and (e) During the past five years, none of the Reporting
Persons nor any of the Funds (as defined below) has been (i) convicted in a criminal proceeding (excluding traffic violations or
similar misdemeanors) or (ii) a party to a civil proceeding of a judicial or administrative body of competent jurisdiction and
as a result of such proceeding was or is subject to a judgment, decree or final order enjoining future violations of, or prohibiting
or mandating activities subject to, federal or state securities laws or finding any violation with respect to such laws.