Security	Date	Code	Shares	Price	A/D	Owned after	D/I	Nature of ownership
Common Stock^{F3,F4,F5,F6,F8,F11,F12}	May 18, 2018	A	3,700	$56.77	A	5,291,874	I	See footnotes
Common Stock^{F3,F4,F6,F7,F8,F11,F12}	May 18, 2018	A	3,700	$56.77	A	45,829,686	I	See footnotes
Common Stock^F1	holding	—	—		—	107,511	D
Common Stock^F2	holding	—	—		—	107,504	D

Table II — derivative securities

Security	Conv. / exercise price	Date	Code	Shares	A/D	Exercisable	Expires	Underlying	Underlying shares	Owned after	D/I
Non-Qualified Stock Options (right to buy)^{F8,F9,F10,F11,F12}	$56.77	May 18, 2018	A	9,260	A	May 18, 2019	May 18, 2028	Common Stock	9,260	9,260	I
Non-Qualified Stock Options (right to buy)^{F8,F9,F11,F12,F13}	$56.77	May 18, 2018	A	9,260	A	May 18, 2019	May 18, 2028	Common Stock	9,260	9,260	I

Explanation of responses

F1Reflects shares of common stock of Seattle Genetics, Inc. (the "Issuer") held directly by Felix J. Baker.
F10As a result of their ownership interest in Baker Biotech Capital (GP), LLC, Felix J. Baker and Julian C. Baker may be deemed to have an indirect pecuniary interest in 9,260 options to purchase the Common Stock of the Issuer beneficially owned 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.
F11The 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.
F12Julian 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.
F13As a result of their ownership interest in Baker Brothers Life Sciences Capital (GP), LLC, Felix J. Baker and Julian C. Baker may be deemed to have an indirect pecuniary interest in 9,260 options to purchase Common Stock of the Issuer beneficially owned 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.
F2Reflects shares of common stock of the Issuer held directly by Julian C. Baker.
F3Restricted Stock Units (the "Restricted Stock Units") payable solely in Common Shares granted to Felix J. Baker on May 18, 2018, pursuant to the Amended and Restated 2007 Equity Incentive Plan. The Restricted Stock Units fully vest on May 18, 2019. Felix J. Baker serves on the Issuer's Board of Directors (the "Board") as a representative of the Funds (as defined below). Pursuant to the policies of Baker Bros. Advisors LP (the "Adviser"), Felix J. Baker does not have a right to any of the Issuer's securities issued as part of his service on the Board and the Funds (as defined below) are entitled to receive all the pecuniary interest in the securities issued. 667, L.P. ("667") and Baker Brothers Life Sciences, L.P. ("Life Sciences")(collectively the "Funds") each owns an indirect proportionate pecuniary interest in the Restricted Stock Units.
F4Solely as a result of Julian C. Baker's and Felix J. Baker's ownership interest in the general partners of the general partners of the Funds, Felix J. Baker and Julian C. Baker may be deemed to have an indirect pecuniary interest in the Restricted Stock Units.
F5After 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.
F6Includes beneficial ownership of 18,100 previously issued restricted stock units payable solely in common shares issued to Felix J. 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 50,000 shares received previously from exercise of 50,000 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.
F7After 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.
F8The disclosure of the grant of Restricted Stock Units and Stock Options (as defined below) reported on this form is a single grant of 3,700 Restricted Stock Units and 9,260 Stock Options. The 3,700 Restricted Stock Units and 9,260 Stock Options (as defined below) are reported for each of the Funds as each has an indirect pecuniary interest.
F9Non-qualified stock options ("Stock Options") convertible solely into Common Shares granted to Felix J. Baker on May 18, 2018, pursuant to the Issuer's Amended and Restated 2007 Equity Incentive Plan. The Stock Options with a strike price of $56.77 vest on May 18, 2019. Felix J. Baker serves on the Board as a representative of the Funds. Pursuant to the policies of the Adviser, Felix J. Baker does not have a right to any of the Issuer's securities issued as part of his service on the Board and the Funds are entitled to receive all the pecuniary interest in the securities issued. The Funds each own an indirect proportionate pecuniary interest in the Stock Options. Solely as a result of Felix J. Baker's and Julian C. Baker's ownership interest in the general partners of the general partners of the Funds, Felix J. Baker and Julian C. Baker may be deemed to have an indirect pecuniary interest in the Stock Options (i.e. no direct 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.