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

UNITED STATES
SECURITIES AND EXCHANGE COMMISSION
Washington, D.C. 20549

SCHEDULE 13D
Under the Securities Exchange Act of 1934

(Amendment No. 25)

Seattle
Genetics, Inc.

(Name of Issuer)

Common Stock, par value $0.001 per share

(Title of Class of Securities)

812578102

(CUSIP Number)

Alexandra A. Toohey

Chief Financial Officer

Baker Bros. Advisors LP
860 Washington Street, 3^rd Floor
New York, NY 10014
(212) 339-5690

(Name, Address and Telephone Number of Person
Authorized to Receive Notices and Communications)

January 31, 2018

(Date of Event which Requires Filing of
this Statement)

If the filing person has previously filed
a statement on Schedule 13G to report the acquisition that is the subject of this Schedule 13D, and is filing this schedule because
of Rule 13d-1(e), 13d-1(f) or 13d-1(g), check the following box. ¨

Note: Schedules filed in paper format
shall include a signed original and five copies of the schedule, including all exhibits. See Rule 13d-7 for other parties to whom
copies are to be sent.

*The remainder of this cover page shall
be filled out for a reporting person’s initial filing on this form with respect to the subject class of securities, and for
any subsequent amendment containing information which would alter disclosures provided in a prior cover page.

The information required on the remainder
of this cover page shall not be deemed to be “filed” for the purpose of Section 18 of the Securities Exchange Act of
1934 (the “Act”) or otherwise subject to the liabilities of that section of the Act but shall be subject to all other
provisions of the Act (however, see the Notes).

(1) Includes 108,520 shares of the Common Stock of Seattle Genetics,
Inc. (the “Issuer”) 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 Securities and Exchange Commission (the “SEC”) on February 1, 2018.

(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.

(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.

(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.

(1) The percentage of ownership is less than 0.1%, 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.

(1) The percentage of ownership is less than 0.1%, 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.

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.

(a) The Reporting Persons are:

(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.

(f) The Adviser is a limited partnership organized under the
laws of the State of Delaware. The Adviser GP is a limited liability company organized under the laws of the State of Delaware.
FBB2 is a limited liability company organized under the laws of the state of Delaware. FBB3 is a limited liability company
organized under the laws of the state of Delaware. The citizenship of each of Julian C. Baker and Felix J. Baker is the United
States of America.

Item
3. Source and Amount of Funds or Other Consideration.

Item 3 of Amendment No. 25 is supplemented and amended, as the
case may be, as follows:

The disclosure in Item 4 below is incorporated herein by reference.

Item 4. Purpose of the Transaction.

Item 4 of Amendment No. 25 is supplemented and superseded, as
the case may be, as follows:

On January 31, 2018, the Issuer entered into an underwriting
agreement (the “Underwriting Agreement”) with J.P. Morgan Securities LLC and and Barclays Capital Inc., as representatives
of the several underwriters listed on Schedule I thereto (the “Underwriters”), related to an underwritten public offering
(the “Offering”) of 11,538,461 shares of the Issuer’s Common Stock at a price to the public of $52.00 per share.
In addition, the Issuer granted the Underwriters an option exercisable for 30 days from the date of the Underwriting Agreement
to purchase, at the public offering price less any underwriting discounts and commissions, up to an additional 1,730,769 shares
of Common Stock to cover overallotments, if any (“Underwriters option”). The Offering is expected to close on February
5, 2018.

Pursuant to the Offering, on February 1, 2018, 667 and Life
Sciences purchased 389,610 shares of Common Stock and 3,456,543 shares of Common Stock, respectively, at the offering price of
$52.00 per share. The Funds purchased the shares of the Issuer’s Common Stock with their working capital.

The Funds hold securities of the Issuer for investment purposes.
The Reporting Persons or their affiliates may purchase additional securities or dispose of securities in varying amounts and at
varying times depending upon the Reporting Persons’ continuing assessments of pertinent factors, including the availability
of shares of Common Stock or other securities for purchase at particular price levels, the business prospects of the Issuer, other
business investment opportunities, economic conditions, stock market conditions, money market conditions, the attitudes and actions
of the board of directors and management of the Issuer, the availability and nature of opportunities to dispose of shares in the
Issuer and other plans and requirements of the particular entities. The Reporting Persons may discuss items of mutual interest
with the Issuer, which could include items in subparagraphs (a) through (j) of Schedule 13D.

Depending upon their assessments of the above factors, the Reporting
Persons or their affiliates may change their present intentions as stated above and they may assess whether to make suggestions
to the management of the Issuer regarding financing, and whether to acquire additional securities of the Issuer, including shares
of Common Stock (by means of open market purchases, privately negotiated purchases, exercise of some or all of the Stock Options
(as defined below), or otherwise) or to dispose of some or all of the securities of the Issuer, including shares of Common Stock,
under their control.

Except as otherwise disclosed herein, at the present time, the
Reporting Persons do not have any plans or proposals with respect to any extraordinary corporate transaction involving the Issuer
including, without limitation, those matters described in subparagraphs (a) through (j) of Item 4 of Amendment No. 25.

(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.

(c) 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.

 (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.

SIGNATURE

After reasonable inquiry and to the best of
my knowledge and belief, I certify that the information set forth in this statement is true, complete and correct.

February 5, 2018