SERMs, an acronym for selective estrogen-receptor modulators, are hormone-related drugs that are chemically very similar to the female hormone, estrogen. Many breast cancer cells are estrogen-dependent, meaning that they depend on estrogen in order to survive and multiply. Because SERMs mimic some of the effects of estrogen, these drugs are able to bind to estrogen receptors on breast cancer cells and block the uptake of estrogen. Without estrogen to stimulate their growth, the estrogen-dependent breast cancer cells “starve” and die. Tamoxifen is the most commonly prescribed drug in the SERM class.