Estrogen receptor is a ligand-activated transcription factor composed of several domains essential for hormone binding, DNA binding, and activation of transcription. Estrogen and its receptors are crucial for sexual development and reproductive function, but also play a role in other tissues such as bone. The Estrogen Receptor is a significant regulator of growth and differentiation in the mammary gland. Estrogen receptors are also implicated in pathological processes including breast cancer, endometrial cancer, and osteoporosis. The presence of estrogen receptors in breast tumors indicates an increased probability of response to anti-estrogen therapy (e.g. tamoxifen).The Estrogen Receptor gene is divided into 8 exons having a genomic length of more than 140kbp.The Estrogen Receptor has 6 functionally separate domains, named A - F. Alternative splicing results in a number of transcript variations, which differ in their 5' UTRs and use different promoters. The protein is restricted to the nucleus where it may form a homodimer or a heterodimer with estrogen receptor 2 Synonyms: Estrogen receptor, ER, Estradiol receptor, ER-alpha, Nuclear receptor subfamily 3 group A member 1, ESR1, ESR, NR3A1, Era, ESRA, DKFZp686N23123, ER-a, Estrogen Receptor alpha