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

(a) and (b) Items 7 through 11 and 13 of each
of the cover pages of this Amendment No. 25 are incorporated herein by reference. Set forth below is the aggregate number and percentage
of shares of Common Stock directly held, as of the date hereof, by each of the following based upon 155,341,860 shares of the Issuer’s
Common Stock that will be outstanding following the Offering plus 1,730,769 shares issued as part of the Underwriter’s option
described in the Issuer’s Prospectus Supplement filed with the SEC on February 1, 2018. Such percentage figures were calculated
in accordance with Rule 13d-3 under the Securities Exchange Act of 1934, as amended.

The Adviser GP, Felix J. Baker and Julian C. Baker as principals
of the Adviser GP, and the Adviser may be deemed to be beneficial owners of securities of the Issuer directly held by the Funds,
and may be deemed to have the power to vote or direct the vote of and the power to dispose or direct the disposition of such securities.

The Reporting Persons disclaim beneficial ownership of the securities
held by each of the Funds, and this Amendment No. 25 shall not be deemed an admission that the Reporting Persons are the beneficial
owners of such securities for purposes of Section 13(d) or for any other purpose, except to the extent that any such Reporting
Persons actually exercises voting or dispositive power with respect to such securities.

Julian C. Baker and Felix J. Baker are also
the sole managers of FBB2 and FBB3 and by policy they do not transact in or vote the securities of the Issuer held by FBB2 and
FBB3.

Felix J. Baker is a Director of the Issuer.
In connection with his service on the Issuer’s Board, Felix J. Baker holds options to purchase Common Stock of the Issuer
(“Stock Options”), Common Stock and restricted stock units payable solely in Common Stock of the Issuer (“Restricted
Stock”) as disclosed in previous amendments to this Schedule 13D.

Felix J. Baker serves on the Issuer’s Board as a representative
of the Funds. The policy of the Funds and the Adviser does not permit employees of the Adviser to receive compensation for serving
as a Director of the Issuer. Therefore, Felix J. Baker has no pecuniary interest in the Stock Options, Common Stock or Restricted
Stock. The Funds are instead entitled to the pecuniary interest in the Stock Options, Common Stock and Restricted Stock.

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 received as directors compensation.
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 received as directors compensation.