Estrogen receptor alpha (ER alpha, NR3A1) is a member of the steroid receptor family. The natural ligand for ER is the classic estrogenic compound 17 beta-estradiol. ER alpha is expressed in a wide variety of tissues such as bones, uterus, bladder, ovary, prostate, testis, epididymis, kidney, breast, heart, vessel wall, pituitary and hypothalamus. The estrogen receptor has a variety of central physiological roles, including those involved in maintenance of the reproductive, cardiovascular, musculoskeletal and central nervous systems. ER alpha has been shown to form homodimers as well as heterodimers with ER beta. Both ER alpha and ER beta can give rise to numerous isoforms.