In working on the development of antiestrogens, scientists have made a somewhat surprising discovery. Some drugs that block the action of estrogen in certain tissues actually can mimic the action of estrogen in other tissues.

Such selectivity is made possible by the fact that the estrogen receptors of different target tissues vary in chemical structure. These differences allow estrogen-like drugs to interact in different ways with the estrogen receptors of different tissues. Such drugs are called selective estrogen receptor modulators, or SERMs, because they selectively stimulate or inhibit the estrogen receptors of different target tissues. For example, a SERM might inhibit the estrogen receptor found in breast cells but activate the estrogen receptor present in uterine endometrial cells. A SERM of this type would inhibit cell proliferation in breast cells, but stimulate the proliferation of uterine endometrial cells.