Human Estrogen Receptor alpha Synthetic Peptide. Estrogen Receptors (ER) are members of the steroid/thyroid hormone receptor superfamily of nuclear receptors. The estrogen receptor is a ligand-activated transcription factor, that when bound to estrogen hormone, induces a conformational change that allows dimerization and binding to estrogen response elements (ERE) in DNA. When bound to EREs, ER can positively or negatively regulate gene transcription through the recruitment of coactivator or corepressor proteins. There are two different forms of the estrogen receptor, alpha and beta, encoded by separate genes (ESR1 and ESR2, respectively). Due to alternative RNA splicing, at least 4 estrogen receptor-alpha isoforms are known to exist (Isofo