Clomiphefei is a drug given to women for use as a fertility aid. It is a SERM (Selective Estrogen Receptor Modulator) which acts by actually binding to the estrogen receptor and thereby blocking estrogen from doing the same. Clearly, this is advantageous when it binds to breast tissue, and prevents estrogen from binding there to cause gynocomastia (although it is not nearly as effective as nolvadex for this purpose).
