Estrogen receptor alpha (ERα), also known as NR3A1 (nuclear receptor subfamily 3, group A, member 1), is one of two main types of estrogen receptor, a nuclear receptor (mainly found as a chromatin-binding protein) that is activated by the sex hormone estrogen. In humans, ERα is encoded by the gene ESR1 (EStrogen Receptor 1). Estrogen receptor-α (ERα) is a key transcription factor in breast cancer and was believed to promote proliferation of breast cancer cells by binding to proximal promoters of key oncogenes.