Estrogen receptor (ER) is a family of hormone receptors activated by estrogen and its derivatives. The two classes of nuclear ER are ER-alpha and ER-beta, in humans encoded by ESR1 and ESR2, respectively. At the amino acid level both types show fairly high similarity. Several isoforms of ER also exist due to alternative splicing. Upon binding to specific ligands, ER translocates into the nucleus and functions as a transcription factor. ERs are highly implicated in cancers, including breast, ovarian, colon, prostate, and endometrial cancer. When choosing an ER antibody, be sure to verify the specifically targeted protein.