13D Filing: Baker Bros. Advisors Buys More Shares of 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. 18)

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).

SCHEDULE
13D

(1) Includes 110,000 shares of the Issuer’s common stock
underlying 110,000 options.

(2) Based
on 139,932,886 shares of common stock as of February 16, 2016 as reported in the Issuer’s 10-K filed with the SEC
on February 19, 2016.

SCHEDULE
13D

(1) Includes 110,000 shares of the Issuer’s common stock
underlying 110,000 options.

(2) Based
on 139,932,886 shares of common stock as of February 16, 2016 as reported in the Issuer’s 10-K filed with the SEC
on February 19, 2016.

SCHEDULE
13D

(1) Includes 110,000 shares of the Issuer’s common stock
underlying 110,000 options.

(2) Based
on 139,932,886 shares of common stock as of February 16, 2016 as reported in the Issuer’s 10-K filed with the SEC
on February 19, 2016.

SCHEDULE
13D

(1) Includes 110,000 shares of the Issuer’s common stock
underlying 110,000 options.

(2) Based
on 139,932,886 shares of common stock as of February 16, 2016 as reported in the Issuer’s 10-K filed with the SEC
on February 19, 2016.

SCHEDULE
13D

(1) The percentage of ownership is less than 0.1%, based
on 139,932,886 shares of common stock as of February 16, 2016 as reported in the Issuer’s 10-K filed with the SEC
on February 19, 2016.

SCHEDULE
13D

(1) The percentage of ownership is less than 0.1%, based
on 139,932,886 shares of common stock as of February 16, 2016 as reported in the Issuer’s 10-K filed with the SEC
on February 19, 2016.

Amendment No. 18 to Schedule 13D

This Amendment No. 18 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”). 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
Adviser has complete and unlimited discretion and authority with respect to the Funds’ investments and voting power over
investments.

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

Item 3 of Schedule 13D is supplemented and amended, as the case
may be, as follows:

The disclosure regarding purchases in Item 5(c) below is incorporated
herein by reference.

Item 4. Purpose of the Transaction.

Item 4 of Schedule 13D is supplemented and superseded, as the
case may be, as follows:

The Funds hold securities of Seattle Genetics, Inc. (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 Schedule 13D.

Item 5. Interest in Securities
of the Issuer.

(a) and (b) Items 7 through 11 and 13 of each
of the cover pages of this Amendment No. 18 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 139,932,886 shares of common
stock as of February 16, 2016 as reported in the Issuer’s 10-K filed with the SEC on February 19, 2016. 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. 18 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 as such may be deemed to be beneficial owners of shares of Common Stock held by FBB2 and
FBB3 and may be deemed to have the power to vote or direct the vote and dispose or direct the disposition of those shares.

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. 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.

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 22, 2016