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ThinkEquity LLC acts as Co-Manager on Seattle Genetics, Inc.'s Follow-On Offering of Common Stock

SAN FRANCISCO, February 7, 2011 — ThinkEquity LLC — A Panmure Gordon Company, a leading investment banking boutique focused on growth companies and their investors, is pleased to announce the completion of a follow-on offering of stock for Seattle Genetics, Inc. on February 7, 2011.

The Company issued 11,500,000 shares of common stock at a price of $15.50 per share raising gross proceeds of approximately $178 million. As part of this offering, the underwriters exercised in full their option to purchase an additional 1,500,000 shares of common stock from the Company. ThinkEquity LLC acted as Co-Manager in this transaction. Seattle Genetics' common stock trades on the NASDAQ Global Select Market under the symbol "SGEN".

Seattle Genetics is a clinical stage biotechnology company focused on the development and commercialization of monoclonal antibody-based therapies for the treatment of cancer and autoimmune diseases.

The securities described above were offered by Seattle Genetics, Inc. pursuant to a shelf registration statement and prospectus supplement filed with and declared effective by the Securities and Exchange Commission. The final prospectus is available at the SEC's website at www.sec.gov, or by contacting ThinkEquity LLC at 600 Montgomery Street, 3rd Floor, San Francisco, CA 94111, Attn: Prospectus Department (415) 249-2935.

ThinkEquity Contact:

Janet Faleski
Managing Director
T: (212) 468-7083
E: jfaleski@thinkequity.com

Media enquiries:

Nathaniel Webb
Group Communications and Investor Relations Director
T: +44 20 7614 8333
E: nathaniel.webb@panmure.com

About ThinkEquity LLC — A Panmure Gordon Company
Focused on the growth sectors of the economy, ThinkEquity provides research, equity financing, M&A advisory, institutional sales and trading, wealth management and asset management services to institutional investors, corporate and private clients, venture capitalists, entrepreneurs and financial sponsors. As a Panmure Gordon company, ThinkEquity accesses the combined financial expertise of professionals based in San Francisco, New York, Boston, Chicago, London, Liverpool and Geneva. Research-driven, we remain true to our founding principles: the power of growth, focus, deep domain expertise and people. www.thinkequity.com

About the Panmure Gordon Group
Founded in 1876, Panmure Gordon & Co. (PMR.L) is a leading independent institutional stockbroker and investment bank. Encompassing its US franchise, ThinkEquity, LLC, the Group employs approximately 240 professionals across the US, the UK, Singapore and Nyon, Switzerland, offering a blend of transatlantic financial services including research, sales and trading, wealth management and asset management. www.panmure.com