Seattle Genetics Inc (SGEN): Baker Bros. Keeps Buying Shares

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

SCHEDULE 13D
Under the Securities Exchange Act of 1934

(Amendment No. 19)

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 100,000 shares of the Issuer’s common stock
underlying 100,000 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 100,000 shares of the Issuer’s common stock
underlying 100,000 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 100,000 shares of the Issuer’s common stock
underlying 100,000 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 100,000 shares of the Issuer’s common stock
underlying 100,000 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. 19 to Schedule 13D

This Amendment No. 19 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 of Schedule 13D is supplemented and amended, as the case
may be, as follows:

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

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

Item 4 of Schedule 13D is supplemented and superseded, as the case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

On March 8, 2016 the Adviser acquired beneficial ownership of 10,000
shares of common stock of Seattle Genetics, Inc. (the “Issuer”), as a result of the exercise of 10,000 options to purchase
the Issuer’s common stock at $4.18 per share (the “Exercised Stock Options”) held directly by Felix J. Baker.
Felix J. Baker currently serves on the Issuer’s Board of Directors (the “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 directors
of the Issuer, and the Funds are instead entitled to the pecuniary interest in the Exercised Stock Options. Felix J. Baker, as
an agent in his capacity as a director of the Issuer, entered into a proceeds agreement (the “Proceeds Agreement”)
with the Adviser on March 8, 2016. Pursuant to the Proceeds Agreement, Felix J. Baker agreed that, with respect to the Exercised
Stock Options and the common stock received as a result of the exercise of the Exercised Stock Options (the “Common Stock”)
on March 8, 2016, the Adviser will have dispositive power as well as the ability to control the timing of exercise of the Exercised
Stock Options and that any proceeds from the sale of the Common Stock will be remitted to the Adviser net of brokerage commissions.
Other than through their control of the Adviser, Felix J. Baker and Julian C. Baker have neither voting nor dispositive power and
have no direct pecuniary interest in the Exercised Stock Options or the Common Stock. Pursuant to the Proceeds Agreement, the Adviser
funded Felix J. Baker’s exercise of the Exercised Stock Options through loans from the Funds (the “Loan Agreements”).
The total amount expended on acquiring the Common Stock was $41,800.

In order to effect the exercise of the Exercised Stock Options,
on March 8, 2016, the Adviser entered into the Loan Agreements with the Funds pursuant to which 667 and Life Sciences loaned $4,322.12
and $37,477.88, respectively, totaling $41,800 to the Adviser for the purpose of acquiring the Common Stock. The loan is due March
8, 2046, or earlier if the Common Stock are sold (“Due Date”), with interest payable through the Due Date at a rate
of 2.33% annually.

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 Schedule 13D.

(a) and (b) Items 7 through 11 and 13 of each
of the cover pages of this Amendment No. 19 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. 19 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 information set forth in Item 4 is hereby incorporated by
reference into this Item 5. The following transactions in the Issuer’s Common Stock were effected by the Funds noted below
during the fifteen 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 $29.00 to $29.94. 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 $28.50 to $28.57. 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.

(3) The reported price is a weighted average
price. These shares were traded in multiple transactions at prices ranging from $29.55 to $30.00. 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.

(4) The reported price is a weighted average price. These shares
were traded in multiple transactions at prices ranging from $29.84 to $30.50. 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.

(5) The reported price is a weighted average price. These shares
were traded in multiple transactions at prices ranging from $30.10 to $30.70. 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.

(6) The reported price is a weighted average price. These shares
were traded in multiple transactions at prices ranging from $30.16 to $30.65. 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.

(7) The reported price is a weighted average price. These shares
were traded in multiple transactions at prices ranging from $31.23 to $31.27. 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.

(8) The reported price is a weighted average price. These shares
were traded in multiple transactions at prices ranging from $32.27 to $33.25. 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.

(9) The reported price is a weighted average price. These shares
were traded in multiple transactions at prices ranging from $31.58 to $31.69. 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.

(10) The reported price is a weighted average price. These shares
were traded in multiple transactions at prices ranging from $32.21 to $32.68. 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.

(11) The reported price is a weighted average price. These shares
were traded in multiple transactions at prices ranging from $33.56 to $33.75. 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.

(12) The reported price is a weighted average price. These shares
were traded in multiple transactions at prices ranging from $33.48 to $33.75. 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.

(13) The reported price is a weighted average price. These shares
were traded in multiple transactions at prices ranging from $32.10 to $32.23. 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.

(14) The reported price is a weighted average price. These shares
were traded in multiple transactions at prices ranging from $32.00 to $32.20. 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.

The disclosure in Item 4 is incorporated
by reference herein.

The Loan Agreement and the Proceeds Agreement are filed as Exhibits
99.1 and 99.2, respectively, and are incorporated by reference herein.

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.

March 8, 2016