Seattle Genetics Inc (SGEN): Baker Bros. Keeps Buying Shares

Page 9 of 15

Page 9 of 15 SEC Filing

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. 19 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 139,932,886 shares of common
stock as of February 16, 2016 as reported in the Issuer’s 10-K filed with the SEC on February 19, 2016. Such percentage figures
were calculated in accordance with Rule 13d-3 under the Securities Exchange Act of 1934, as amended.

Holder Number of
Shares
Percentage of Class Outstanding
667, L.P. 4,240,731 3.0 %
Baker Brothers Life Sciences, L.P. 36,789,568 26.3 %

The Adviser GP, Felix J. Baker and Julian C. Baker as principals
of the Adviser GP, and the Adviser may be deemed to be beneficial owners of securities of the Issuer directly held by the Funds,
and may be deemed to have the power to vote or direct the vote of and the power to dispose or direct the disposition of such securities.

Page 9 of 15 Pages

Follow Seagen Inc. (NASDAQ:SGEN)

Page 9 of 15