Tamoxifen is an antagonist of the estrogen receptor in breast tissue via its active metabolite, hydroxytamoxifen. In other tissues such as the endometrium, it behaves as an agonist, and thus may be characterized as a mixed agonist/antagonist. Tamoxifen is the usual endocrine (anti-estrogen) therapy for hormone receptor-positive breast cancer in pre-menopausal women, and is also a standard in post-menopausal women although aromatase inhibitors are also frequently used in that setting.

Some breast cancer cells require estrogen to grow. Estrogen binds to and activates the estrogen receptor in these cells. Tamoxifen is metabolized into compounds that also bind to the estrogen receptor but do not activate it. Because of this competitive antagonism, tamoxifen acts like a key broken off in the lock that prevents any other key from being inserted, preventing estrogen from binding to its receptor. Hence breast cancer cell growth is blocked.

Tamoxifen was discovered by pharmaceutical company Imperial Chemical Industries (now AstraZeneca) and is sold under the trade names Nolvadex, Istubal, and Valodex. However, the drug, even before its patent expiration, was and still is widely referred to by its generic name "tamoxifen."

Frederick Charles Hutchinson (August 12, 1919 – November 12, 1964) was an American professional baseball player, a major league pitcher for the Detroit Tigers. He also was a manager for three major league teams. Stricken with fatal lung cancer at the height of his managerial career as leader of the pennant-contending Cincinnati Reds, he was commemorated one year after his death when his brother, Dr. William Hutchinson, created the Fred Hutchinson Cancer Research Center as a division of the Pacific Northwest Research Foundation, in the Hutchinson's native city of Seattle, Washington. The FHCRC, which became independent in 1972, is now one of the best-known facilities of its kind in the world.

Fred Hutchinson, known throughout baseball as “Hutch,” attended the University of Washington, where he was a 1939 initiate of Alpha Sigma Phi Fraternity. A right-handed pitcher, he entered the organized baseball ranks in 1938 with the unaffiliated Seattle Rainiers of the AA Pacific Coast League and caused an immediate sensation at age 19, winning a league-best 25 games and that season’s Minor League Player of the Year award as bestowed by The Sporting News.