faslodex

FASLODEX works by binding to these estrogen receptors, which blocks estrogen from binding. FASLODEX also causes the estrogen receptor to change shape and not work as well. In addition, FASLODEX causes a decrease (downregulation) in the number of estrogen receptors. This may interfere with cancer cell activity, which, in part, may slow the growth of the breast cancer.
http://www.faslodex.com/faslodex-what-is/index.aspx