The estrogen receptor alpha (ERalpha, UniProt/Swiss-Prot entry P03372) belongs to the family of nuclear hormone receptors, which are ligand-activated transcription factors. They are important for the regulation of gene expression, cellular proliferation and differentiation, sexual development and reproductive function. Estrogen receptors are also involved in pathological processes such as breast cancer, and osteoporosis. ERalpha can regulate transcription by direct binding to estrogen response elements (EREs) in the DNA or by interaction with other transcription factors. It may also form a heterodimer with ERbeta.