Security	Date	Code	Shares	Price	A/D	Owned after	D/I	Nature of ownership
Common Stock^{F11,F6,F7,F12}	Apr 30, 2018	P	154,218	$50.9013	A	44,509,553	I	See footnotes
Common Stock^{F13,F6,F7,F12}	Apr 30, 2018	P	248,121	$51.0278	A	44,757,674	I	See footnotes
Common Stock^{F14,F6,F7,F12}	Apr 30, 2018	P	52,700	$51.0659	A	44,810,374	I	See footnotes
Common Stock^{F15,F6,F7,F12}	Apr 30, 2018	P	168,097	$51.2489	A	44,978,471	I	See footnotes
Common Stock^{F3,F4,F6,F7,F8,F10}	May 1, 2018	M	10,000	$10.08	A	5,288,174	I	See footnotes
Common Stock^{F4,F5,F6,F7,F8,F10}	May 1, 2018	M	10,000	$10.08	A	44,988,471	I	See footnotes
Common Stock^F6,F7,F12	May 1, 2018	P	100,000	$51.14	A	45,088,471	I	See footnotes
Common Stock^{F16,F6,F7,F12}	May 1, 2018	P	72,533	$51.2552	A	45,161,004	I	See footnotes
Common Stock^{F17,F6,F7,F12}	May 1, 2018	P	22,700	$51.2962	A	45,183,704	I	See footnotes
Common Stock^F6,F7,F12	May 1, 2018	P	10,500	$51.36	A	45,194,204	I	See footnotes
Common Stock^{F18,F6,F7,F12}	May 1, 2018	P	57,270	$51.3656	A	45,251,474	I	See footnotes
Common Stock^{F19,F6,F7,F12}	May 1, 2018	P	37,963	$52.0563	A	45,289,437	I	See footnotes
Common Stock^{F20,F6,F7,F12}	May 1, 2018	P	5,951	$52.3303	A	45,295,388	I	See footnotes
Common Stock^F6,F7,F12	May 2, 2018	P	2,000	$51.875	A	45,297,388	I	See footnotes
Common Stock^{F21,F6,F7,F12}	May 2, 2018	P	37,400	$52.3268	A	45,334,788	I	See footnotes
Common Stock^{F22,F6,F7,F12}	May 2, 2018	P	21,442	$52.2608	A	45,356,230	I	See footnotes
Common Stock^{F23,F6,F7,F12}	May 2, 2018	P	9,105	$54.4193	A	45,365,335	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)^F9,F3,F10	$10.08	Apr 30, 2018	M	10,000	D	May 15, 2009	May 16, 2018	Common Stock	10,000	0	I
Non-Qualified Stock Options (right to buy)^F9,F5,F10	$10.08	Apr 30, 2018	M	10,000	D	May 15, 2009	May 16, 2018	Common Stock	10,000	0	I

Explanation of responses

F1Reflects shares of common stock of Seattle Genetics, Inc. (the "Issuer") held directly by Felix J. Baker.
F10The disclosure of the exercise of directors options for shares of common stock reported on this form is a single exercise of 10,000 shares. The 10,000 shares are reported for each of the Funds as each has an indirect pecuniary interest.
F11The price reported in Column 4 is a weighted average price. These shares were traded in multiple transactions at prices ranging from $50.71 to $51.00, inclusive. The reporting persons undertake to provide to the Issuer, any security holder of the Issuer, or the staff of the Securities and Exchange Commission ("SEC"), upon request, full information regarding the number of shares traded at each separate price within the range set forth in this footnote.
F12After 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.
F13The price reported in Column 4 is a weighted average price. These shares were traded in multiple transactions at prices ranging from $50.91 to $51.40, inclusive. The reporting persons undertake to provide to the Issuer, any security holder of the Issuer, or the staff of the SEC, upon request, full information regarding the number of shares traded at each separate price within the range set forth in this footnote.
F14The price reported in Column 4 is a weighted average price. These shares were traded in multiple transactions at prices ranging from $50.67 to $51.47, inclusive. The reporting persons undertake to provide to the Issuer, any security holder of the Issuer, or the staff of the SEC, upon request, full information regarding the number of shares traded at each separate price within the range set forth in this footnote.
F15The price reported in Column 4 is a weighted average price. These shares were traded in multiple transactions at prices ranging from $51.01 to $52.00, inclusive. The reporting persons undertake to provide to the Issuer, any security holder of the Issuer, or the staff of the SEC, upon request, full information regarding the number of shares traded at each separate price within the range set forth in this footnote.
F16The price reported in Column 4 is a weighted average price. These shares were traded in multiple transactions at prices ranging from $51.07 to $51.46, inclusive. The reporting persons undertake to provide to the Issuer, any security holder of the Issuer, or the staff of the SEC, upon request, full information regarding the number of shares traded at each separate price within the range set forth in this footnote.
F17The price reported in Column 4 is a weighted average price. These shares were traded in multiple transactions at prices ranging from $51.10 to $51.72, inclusive. The reporting persons undertake to provide to the Issuer, any security holder of the Issuer, or the staff of the SEC, upon request, full information regarding the number of shares traded at each separate price within the range set forth in this footnote.
F18The price reported in Column 4 is a weighted average price. These shares were traded in multiple transactions at prices ranging from $51.27 to $51.46, inclusive. The reporting persons undertake to provide to the Issuer, any security holder of the Issuer, or the staff of the SEC, upon request, full information regarding the number of shares traded at each separate price within the range set forth in this footnote.
F19The price reported in Column 4 is a weighted average price. These shares were traded in multiple transactions at prices ranging from $51.54 to $52.50, inclusive. The reporting persons undertake to provide to the Issuer, any security holder of the Issuer, or the staff of the SEC, upon request, full information regarding the number of shares traded at each separate price within the range set forth in this footnote.
F2Reflects shares of common stock of the Issuer held directly by Julian C. Baker.
F20The price reported in Column 4 is a weighted average price. These shares were traded in multiple transactions at prices ranging from $52.00 to $52.50, inclusive. The reporting persons undertake to provide to the Issuer, any security holder of the Issuer, or the staff of the SEC, upon request, full information regarding the number of shares traded at each separate price within the range set forth in this footnote.
F21The price reported in Column 4 is a weighted average price. These shares were traded in multiple transactions at prices ranging from $51.91 to $52.88, inclusive. The reporting persons undertake to provide to the Issuer, any security holder of the Issuer, or the staff of the SEC, upon request, full information regarding the number of shares traded at each separate price within the range set forth in this footnote.
F22The price reported in Column 4 is a weighted average price. These shares were traded in multiple transactions at prices ranging from $51.88 to $52.76, inclusive. The reporting persons undertake to provide to the Issuer, any security holder of the Issuer, or the staff of the SEC, upon request, full information regarding the number of shares traded at each separate price within the range set forth in this footnote.
F23The price reported in Column 4 is a weighted average price. These shares were traded in multiple transactions at prices ranging from $54.40 to $54.42, inclusive. The reporting persons undertake to provide to the Issuer, any security holder of the Issuer, or the staff of the SEC, upon request, full information regarding the number of shares traded at each separate price within the range set forth in this footnote.
F3As 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 10,000 shares of Common Stock of the Issuer received upon the exercise of stock options of the Issuer held by 667, L.P. ("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. The policy of the Funds (defined below) and Baker Bros. Advisors LP (the "Adviser"), does not permit full-time employees or principals of the Adviser to receive compensation for serving as a Director of the Issuer.
F4The shares of common stock were received upon exercise of 10,000 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 shares of common stock received upon exercise of such stock options. Each of the Funds owns an indirect proportionate pecuniary interest in the shares of 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. Solely as a result of their 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 shares issued upon exercise of the stock options (i.e. no direct pecuniary interest).
F5As 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 10,000 shares of Common Stock of the Issuer received upon the exercise of stock options of the Issuer held by Baker Brothers Life Sciences, L.P. ("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. The policy of the Funds (defined below) and the Adviser, does not permit full-time employees or principals of the Adviser to receive compensation for serving as a Director of the Issuer.
F6Includes 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 50,000 shares, 40,000 of which were 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.
F7The Adviser serves as the Investment Adviser to Life Sciences and 667 (collectively 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.
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.
F9These non-qualified stock options are not priced.

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.