Estrogen Receptor α (ERα) is a 65 kDa nuclear hormone receptor. This steroid hormone receptor is involved in the regulation of eukaryotic gene expression and affects cellular proliferation and differentiation in target tissues. ERα is a ligand-activated transcription factor and upon being bound by a ligand such as estrogen, a conformational change occurs that allows dimerization and binding to an estrogen response element. When bound to DNA, Estrogen Receptor α can positively or negatively regulate gene transcription. Like other steroid hormone receptors, Estrogen Receptors are intracellular proteins. Two ERs are known, ERα, classically known as ER, and ERβ. The Estrogen Receptor is believed to play an important role in the growth and development of a subset of hormone dependent human breast cancers. ERβ is expressed in prostate, ovary, and other reproductive organs. Although its steroid specificity is very similar to that of the ERα, the molecular mechanisms regulating the transcripti