13D Filing: Baker Bros. Advisors Buys More Shares of Seattle Genetics Inc (SGEN)

The Adviser has voting and investment power over the Stock Options,
Common Stock underlying such Stock Options, Restricted Stock and Common Stock held by Felix Baker. The Adviser GP, and Felix J.
Baker and Julian C. Baker as principals of the Adviser GP, may be deemed to have the power to vote or direct the vote of and the
power to dispose or direct the disposition of the Stock Options, Common Stock underlying such Stock Options, Restricted Stock and
Common Stock of the Issuer held by Felix J. Baker.

(c) The following transactions in the Issuer’s
Common Stock were effected by the Funds noted below during the five days preceding the filing of this statement using working capital
of the applicable purchasing Fund. All purchase transactions were effected in the open market directly with a broker-dealer. Except
as disclosed herein or in any previous amendments to this Schedule 13D, none of the Reporting Persons or their affiliates has effected
any other transactions in securities of the Issuer during the past 60 days.

(1) The reported price is a weighted average price. These shares
were traded in multiple transactions at a prices ranging from $29.42 to $30.15. The Reporting Persons undertake to provide the
Issuer, any security holder of the Issuer, or the staff of the Securities and Exchange Commission (the “Staff”), upon
request, full information regarding the number of shares traded at each separate price within the ranges set forth in this footnote.

(2) The reported price is a weighted
average price. These shares were traded in multiple transactions at prices ranging from $30.01 to $30.48. The Reporting Persons
undertake to provide the Issuer, any security holder of the Issuer, or the Staff, upon request, full information regarding the
number of shares traded at each separate price within the ranges set forth in this footnote.

(d) Certain securities
of the Issuer are held directly by 667, a limited partnership the sole general partner of which is Baker Biotech Capital, L.P.,
a limited partnership the sole general partner of which is Baker Biotech Capital (GP), LLC. Julian C. Baker and Felix J. Baker
are the controlling members of Baker Biotech Capital (GP), LLC.

Certain securities of the Issuer are held directly by Life Sciences,
a limited partnership the sole general partner of which is Baker Brothers Life Sciences Capital, L.P., a limited partnership the
sole general partner of which is Baker Brothers Life Sciences Capital (GP), LLC. Julian C. Baker and Felix J. Baker are the controlling
members of Baker Brothers Life Sciences Capital (GP), LLC.

(e) Not applicable.