13D Filing: Baker Bros. Advisors and Seattle Genetics Inc (SGEN)

In order to effect the exercise of the Exercised Stock Options,
on May 15, 2017, the Adviser entered into the Loan Agreements with the Funds pursuant to which 667 and Life Sciences loaned $10,792
and $91,208, respectively, totaling $102,000 to the Adviser for the purpose of acquiring the Common Stock. The loan is due May
15, 2047, or earlier if the Common Stock are sold (“Due Date”), with interest payable through the Due Date at a rate
of 2.75% annually.

The Funds hold securities of the Issuer for investment purposes.
The Reporting Persons or their affiliates may purchase additional securities or dispose of securities in varying amounts and at
varying times depending upon the Reporting Persons’ continuing assessments of pertinent factors, including the availability
of shares of Common Stock or other securities for purchase at particular price levels, the business prospects of the Issuer, other
business investment opportunities, economic conditions, stock market conditions, money market conditions, the attitudes and actions
of the board of directors and management of the Issuer, the availability and nature of opportunities to dispose of shares in the
Issuer and other plans and requirements of the particular entities. The Reporting Persons may discuss items of mutual interest
with the Issuer, which could include items in subparagraphs (a) through (j) of Schedule 13D.

Depending upon their assessments of the above factors, the Reporting
Persons or their affiliates may change their present intentions as stated above and they may assess whether to make suggestions
to the management of the Issuer regarding financing, and whether to acquire additional securities of the Issuer, including shares
of Common Stock (by means of open market purchases, privately negotiated purchases, exercise of some or all of the Stock Options
(as defined below), or otherwise) or to dispose of some or all of the securities of the Issuer, including shares of Common Stock,
under their control.

Except as otherwise disclosed herein, at the present time, the Reporting
Persons do not have any plans or proposals with respect to any extraordinary corporate transaction involving the Issuer including,
without limitation, those matters described in subparagraphs (a) through (j) of Item 4 of Schedule 13D.

Item 5. Interest in Securities
of the Issuer.

(a) and (b) Items 7 through 11 and 13 of each
of the cover pages of this Amendment No. 24 are incorporated herein by reference. Set forth below is the aggregate number and percentage
of shares of Common Stock directly held, as of the date hereof, by each of the following based upon 142,716,597 shares of common
stock as of April 26, 2017 as reported in the Issuer’s 10-Q filed with the SEC on May 1, 2017. Such percentage figures were
calculated in accordance with Rule 13d-3 under the Securities Exchange Act of 1934, as amended.