Loan Agreement Between Baker Brothers Life Sciences, L.P., 667, L.P., and Baker Bros. Advisors LP

Exhibit 99.1

Loan Agreement Between Xxxxx Brothers Life Sciences, L.P., 667, L.P., and Xxxxx Bros. Advisors LP

For value received, Xxxxx Bros. Advisors LP (the “Management Company”) promises to pay Xxxxx Brothers Life Sciences, L.P., and 667, L.P., (collectively “The Funds”), the amounts set forth in Schedule A below, payable on the Due Date (as defined below) with interest payable through the Due Date at a rate of 1.44% annually.

The Funds are lending the Management Company these amounts so that Xxxxx Xxxxx, as agent of the Management Company, may exercise 17,500 Seattle Genetics, Inc. (“SGEN”) Director’s Non-qualified Stock Options at $12.76 per share (the “Shares”) and deposit the Shares into Xxxxx Xxxxx’x brokerage account held at JPMorgan.

The “Due Date” shall be April 23, 2050, however, following the sale by Xxxxx Xxxxx of all of the Shares, the Due Date shall accelerate to the date that is 10 days after date of the last sale of Shares.

This Loan Agreement shall be construed in accordance with the laws of the State of New York.

Xxxxx Bros. Advisors LP

/s/ Xxxxxxxxx X. Xxxxxx

By: Xxxxxxxxx X. Xxxxxx, Chief Financial Officer

667, L.P.

By: Xxxxx Bros. Advisors LP, management company and investment adviser to 667, L.P., pursuant to authority granted to it by Xxxxx Biotech Capital, L.P., general partner to 667, L.P., and not as the general partner.

By:	/s/ Xxxxx X. Xxxxxxx
Xxxxx Xxxxxxx
President

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XXXXX BROTHERS LIFE SCIENCES, L.P.

By: XXXXX BROS. ADVISORS LP, , management company and investment adviser to Xxxxx Brothers Life Sciences, L.P., pursuant to authority granted to it by Xxxxx Brothers Life Sciences Capital, L.P., general partner to Xxxxx Brothers Life Sciences, L.P., and not as the general partner.

By:	/s/ Xxxxx X. Xxxxxxx
Xxxxx Xxxxxxx
President

Schedule A

Fund			667, X.X.				Xxxxx Brothers Life Sciences, L.P.				Total
Loan			$	18,845			$	204,455			$	223,300

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