Sec Form 4 Filing - BAKER BROS. ADVISORS LP @ SEATTLE GENETICS INC /WA - 2020-04-23

FORM 4 Check this box if no longer subject to Section 16. Form 4 or Form 5 obligations may continue. See Instruction 1(b).

UNITED STATES SECURITIES AND EXCHANGE COMMISSION Washington, D.C. 20549 STATEMENT OF CHANGES IN BENEFICIAL OWNERSHIP Filed pursuant to Section 16(a) of the Securities Exchange Act of 1934, Section 17(a) of the Public Utility Holding Company Act of 1935 or Section 30(h) of the Investment Company Act of 1940

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1. Name and Address of Reporting Person BAKER BROS. ADVISORS LP 2. Issuer Name and Ticker or Trading Symbol SEATTLE GENETICS INC /WA [ SGEN] 5. Relationship of Reporting Person(s) to Issuer (Check all applicable) X __ Director X __ 10% Owner _____ Officer (give title below) _____ Other (specify below) (Last) (First) (Middle) 860 WASHINGTON STREET, 3RD FLOOR 3. Date of Earliest Transaction (MM/DD/YY) 04/23/2020 (Street) NEW YORK, NY10014 4. If Amendment, Date Original Filed (MM/DD/YY) 6. Individual or Joint/Group Filing (Check Applicable Line) _____ Form filed by One Reporting Person __ X __ Form filed by More than One Reporting Person (City) (State) (Zip)

Table I - Non-Derivative Securities Acquired, Disposed of, or Beneficially Owned 1.Title of Security (Instr. 3) 2. Transaction Date (MM/DD/YY) 2A. Deemed Execution Date, if any (MM/DD/YY) 3. Transaction Code (Instr. 8) 4. Securities Acquired (A) or Disposed of (D) (Instr. 3, 4 and 5) 5. Amount of Securities Beneficially Owned Following Reported Transaction(s) (Instr. 3 and 4) 6. Ownership Form: Direct (D) or Indirect (I) (Instr. 4) 7. Nature of Indirect Beneficial Ownership (Instr. 4) Code V Amount (A) or (D) Price Common Stock 181,740 ( 1 ) D Common Stock 181,733 ( 2 ) D Common Stock 04/23/2020 M 17,500 A $ 12.76 4,319,854 I See footnotes ( 3 ) ( 4 ) ( 5 ) ( 6 ) ( 8 ) ( 9 ) ( 11 ) Common Stock 04/23/2020 M 17,500 A $ 12.76 45,860,149 I See footnotes ( 4 ) ( 5 ) ( 6 ) ( 7 ) ( 8 ) ( 9 ) ( 11 )

Table II - Derivative Securities Acquired, Disposed of, or Beneficially Owned ( e.g. , puts, calls, warrants, options, convertible securities) 1. Title of Derivative Security (Instr. 3) 2. Conversion or Exercise Price of Derivative Security 3. Transaction Date (MM/DD/YY) 3A. Deemed Execution Date, if any (MM/DD/YY) 4. Transaction Code (Instr. 8) 5. Number of Derivative Securities Acquired (A) or Disposed of (D) (Instr. 3, 4, and 5) 6. Date Exercisable and Expiration Date (MM/DD/YY) 7. Title and Amount of Underlying Securities (Instr. 3 and 4) 8. Price of Derivative Security (Instr. 5) 9. Number of Derivative Securities Beneficially Owned Following Reported Transaction(s) (Instr. 4) 10. Ownership Form of Derivative Security: Direct (D) or Indirect (I) (Instr. 4) 11. Nature of Indirect Beneficial Ownership (Instr. 4) Code V (A) (D) Date Exercisable Expiration Date Title Amount or Number of Shares Non-Qualified Stock Options (right to buy) $ 12.76 04/23/2020 M 17,500 05/20/2011 05/21/2020 Common Stock 17,500 ( 10 ) 0 I See Footnotes ( 3 ) ( 11 ) Non-Qualified Stock Options (right to buy) $ 12.76 04/23/2020 M 17,500 05/20/2011 05/21/2020 Common Stock 17,500 ( 10 ) 0 I See Footnotes ( 7 ) ( 11 )

Reporting Owners Reporting Owner Name / Address Relationships Director 10% Owner Officer Other BAKER BROS. ADVISORS LP 860 WASHINGTON STREET 3RD FLOOR NEW YORK, NY10014 X X 667, L.P. 860 WASHINGTON STREET, 3RD FLOOR NEW YORK, NY10014 X X Baker Bros. Advisors (GP) LLC 860 WASHINGTON STREET 3RD FLOOR NEW YORK, NY10014 X X Baker Brothers Life Sciences LP 860 WASHINGTON STREET 3RD FLOOR NEW YORK, NY10014 X X BAKER FELIX 860 WASHINGTON STREET 3RD FLOOR NEW YORK, NY10014 X X BAKER JULIAN 860 WASHINGTON STREET, 3RD FLOOR NEW YORK, NY10014 X X Signatures By: Baker Bros. Advisors LP, Name: Scott L. Lessing, Title: President /s/ Scott L. Lessing 04/24/2020 Signature of Reporting Person Date Baker Bros. Advisors LP, Mgmt. Co. and Inv. Adviser to 667, L.P., pursuant to authority granted by Baker Biotech Capital, L.P., GP to 667, L.P. Name: Scott L. Lessing, Title: President /s/ Scott L. Lessing 04/24/2020 Signature of Reporting Person Date By: Baker Bros. Advisors (GP) LLC, Name: Scott L. Lessing, Title: President /s/ Scott L. Lessing 04/24/2020 Signature of Reporting Person Date Baker Bros. Advisors LP, Mgmt. Co. and Inv. Adviser to BAKER BROTHERS LIFE SCIENCES, L.P., pursuant to authority granted by Baker Brothers Life Sciences Capital, L.P., GP to Baker Brothers Life Sciences, L.P., Name: Scott L. Lessing, Title: President /s/ 04/24/2020 Signature of Reporting Person Date /s/ Felix J. Baker 04/24/2020 Signature of Reporting Person Date /s/ Julian C. Baker 04/24/2020 Signature of Reporting Person Date

Explanation of Responses:

( 1 )Reflects shares of common stock ("Common Stock") of Seattle Genetics, Inc. (the "Issuer") held directly by Felix J. Baker. ( 2 )Reflects Common Stock held directly by Julian C. Baker. ( 3 )After giving effect to the transaction reported herein and as a result of their ownership interest in (i) Baker Biotech Capital, L.P. and (ii) 667, L.P. ("667"), Julian C. Baker and Felix J. Baker each may be deemed to have an indirect pecuniary interest in Common Stock reported in column 5 of Table I directly held by 667, a limited partnership of which the sole general partner is Baker Biotech Capital, L.P., a limited partnership of which the sole general partner is Baker Biotech Capital (GP),LLC, due to their interest in 667 and Baker Biotech Capital, L.P.'s right to receive an allocation of a portion of the profits from 667. The policy of the Funds (defined below) and Baker Bros. Advisors LP (the "Adviser"), does not permit full-time employees of the Adviser or managing members of Baker Bros. Advisors (GP) LLC (the "Adviser GP") to receive compensation for serving as a director of the Issuer. ( 4 )The Adviser serves as the Investment Adviser to 667 and Baker Brothers Life Sciences, L.P. ("Life Sciences", and together with 667, the "Funds"). In connection with the services provided by the Adviser, the Adviser receives an asset-based management fee that does not confer any pecuniary interest in the securities held by the Funds. The Adviser GP is the Adviser's sole general partner. Julian C. Baker and Felix J. Baker are managing members of the Adviser GP. The Adviser has complete and unlimited discretion and authority with respect to the investment and voting power of the securities held by the Funds. The general partners of the Funds relinquished to the Adviser all discretion and authority with respect to the investment and voting power of the securities held by the Funds. ( 5 )Common Stock received upon exercise of 17,500 options exercisable into Common Stock on a 1-for1 basis ("Stock Options") that were issued to Felix J. Baker in his capacity as a director of the Issuer. Felix J. Baker, pursuant to the policies of the Adviser, does not have any right to the pecuniary interest in the Stock Options issued for his service on the Board of Directors of the Issuer (the "Board") or the Common Stock received upon exercise of such Stock Options. Each of the Funds owns an indirect proportionate pecuniary interest in the Common Stock received upon exercise of the Stock Options issued in connection with Felix J. Baker's service on the Board less the exercise cost of those options. ( 6 )Solely as a result of their ownership interest in (i) the general partners of the Funds and (ii) the Funds as limited partners, Felix J. Baker and Julian C. Baker may be deemed to have an indirect pecuniary interest in the shares issued upon exercise of the stock options (i.e. no direct pecuniary interest). ( 7 )After giving effect to the transaction reported herein and as a result of their ownership interest in ( i) Baker Brothers Life Sciences Capital, L.P. and (ii) Life Sciences, Julian C. Baker and Felix J. Baker each may be deemed to have an indirect pecuniary interest in Common Stock reported in column 5 of Table I directly held by Life Sciences, a limited partnership of which the sole general partner is Baker Brothers Life Sciences Capital, L.P., a limited partnership of which the sole general partner is Baker Brothers Life Sciences Capital (GP), LLC, due to their interest in Life Sciences and Baker Brothers Life Sciences Capital, L.P.'s right to receive an allocation of a portion of the profits from Life Sciences. The policy of the Funds and the Adviser, does not permit full-time employees of the Adviser or managing members of the Adviser GP to receive compensation for serving as a director of the Issuer. ( 8 )Includes beneficial ownership of 24,763 previously issued restricted stock units payable solely in Common Stock issued to Felix Baker in his capacity as a director of the Issuer pursuant to the Amended and Restated 2007 Equity Incentive Plan of which the Funds may be deemed to own a portion and 77,500 shares, 60,000 of which were received previously, from exercise of 77,500 Stock Options of the Issuer that were issued to Felix J. Baker in his capacity as a director of the Issuer, of which the Funds may be deemed to own a portion. ( 9 )Julian C. Baker, Felix J. Baker, the Adviser GP and the Adviser disclaim beneficial ownership of the securities held directly by the Funds except to the extent of their pecuniary interest therein, and this report shall not be deemed an admission that any of Julian C. Baker, Felix J. Baker, the Adviser GP or the Adviser is a beneficial owner of such securities for purposes of Section 16 or any other purpose. ( 10 )These non-qualified stock options are not priced. ( 11 )The disclosure of the exercise of Stock Options for Common Stock reported on this form is a single exercise of 17,500 shares. The 17,500 shares are reported for each of the Funds as each has an indirect pecuniary interest.

Remarks: Felix J. Baker is a director of Seattle Genetics, Inc. (the "Issuer"). By virtue of his representation on the Board of Directors of the Issuer, for purposes of Section 16 of the Securities Exchange Act of 1934, as amended, the reporting persons are deemed directors by deputization of the Issuer.

Reminder: Report on a separate line for each class of securities beneficially owned directly or indirectly.

* If the form is filed by more than one reporting person, see Instruction 4(b)(v).

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