ER alpha (Estrogen receptor alpha; also Estradiol receptor and NR3A1) is a 65-70kD member of the NR3 subfamily, nuclear hormone receptor family of proteins. It is widely expressed, and serves as a strong activator of estrogen-responsive genes. ER-a is normally quiescent and bound to heat-shock proteins and immunophilins. Following b-estradiol binding, it becomes activated, either homodimerizes or heterodimerizes with ER-b, and binds to DNA with multiple co-activators. Major phosphorylation sites exist at Tyr537, Ser167 and Ser118. Phosphorylation at Ser118 regulates ER-alpha