Estrogen has been thought to be required for the development, differentiation and function of the female reproductive tract (George & Wilson 1988). It promotes the proliferation of the uterine and vaginal epithelium and ovarian follicular development (Canez et al. 1992); the development of the mammary gland, external genitalia, appropriate reproductive sexual behavior, and other secondary sexual characteristics (George & Wilson 1988). In more non-traditional target tissues, estrogen action has been shown to influence bone development, lipid metabolism and cardiovascular function (Auchus & Fuqua 1994, Ciocca & Vargas Roig 1995).
The biological actions of estrogens, particularly 17β-estradiol(estradiol), are mediated through the estrogen receptor, which functions as a ligand-inducible transcription factor and is a member of the nuclear receptor superfamily that bind steroids, thyroid hormone, retinoids, prostanoids and vitamin D1 (Tsai & O'Malley 1994). Like other nuclear receptors, the estrogen receptor is composed of distinct functional domains (Fig. 1)