Biological effects of steroids are mediated through an initial interaction with specific receptor belonging to a member of the steroid/thyroid/retinoid receptor gene superfamily (1 ,2 ). Recently, increasing numbers of new members of this family, including their subtypes have been cloned and characterized. For instance, until the recent cloning of a second estrogen receptor (ER) α, ERβ had been considered as the only receptor able to bind estrogen with high affinity. ERα protein is smaller than ERβ protein but has a similar high affinity for estradiol, as does the receptor (3 ,4 ). Both receptors demonstrate high conservation of amino acid sequence in regions of the hormone-binding domain known to be important in contacting ligands (3 ). However, tissue distribution and relative expression levels of ERα have been demonstrated to be different from those of ERβ, which suggests possible different biological roles of ERα in mammalian estrogen dependent tissues (5 ).
