Security	Date	Code	Shares	Price	A/D	Owned after	D/I	Nature of ownership
Common Stock^{F3,F4,F5,F7,F8}	Feb 1, 2018	P	389,610	$52.00	A	5,278,174	I	See footnote
Common Stock^{F3,F5,F6,F7,F8}	Feb 1, 2018	P	3,456,543	$52.00	A	44,355,335	I	See footnotes
Common Stock^F1	holding	—	—		—	107,511	D
Common Stock^F2	holding	—	—		—	107,504	D

Table II — derivative securities

No Table II lines on this filing.

Explanation of responses

F1Reflects shares of common stock of Seattle Genetics, Inc. (the "Issuer") held directly by Felix J. Baker.
F2Reflects shares of common stock of the Issuer held directly by Julian C. Baker.
F3On February 1, 2018, 667, L.P. ("667") and Baker Brothers Life Sciences, L.P. ("Life Sciences", and together with 667, the "Funds") purchased 389,610 and 3,456,543 shares of the Common Stock, respectively of Seattle Genetics, Inc. (the "Issuer"), pursuant to an underwritten public offering.
F4After giving effect to the transactions reported herein and as a result of their ownership interest in Baker Biotech Capital (GP), LLC, Julian C. Baker and Felix J. Baker each may be deemed to have an indirect pecuniary interest in the Issuer's shares of 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 Baker Biotech Capital, L.P.'s right to receive an allocation of a portion of the profits from 667.
F5Includes beneficial ownership of 18,100 previously issued restricted stock units payable solely in common shares 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 fund may be deemed to own a portion and 40,000 shares received previously from exercise of 40,000 stock options of the Issuer that were issued to Felix J. Baker in his capacity as a director of the Issuer, of which the fund may be deemed to own a portion.
F6After giving effect to the transactions reported herein and as a result of their ownership interest in Baker Brothers Life Sciences Capital (GP), LLC, Julian C. Baker and Felix J. Baker each may be deemed to have an indirect pecuniary interest in the Issuer's shares of 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 Baker Brothers Life Sciences Capital, L.P.'s right to receive an allocation of a portion of the profits from Life Sciences.
F7Baker Bros. Advisors LP (the "Adviser") serves as the Investment Adviser to 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. Baker Bros. Advisors (GP) LLC (the "Adviser GP") is the Adviser's sole general partner. Julian C. Baker and Felix J. Baker are principals 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. Pursuant to agreements between Felix J. Baker and the Adviser, the Adviser has investment and dispositive power over the Stock Options and any shares received as a result of the exercise of options.
F8Julian 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.

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.