Estrogen and progesterone receptors are members of a family transcription factors regulated by the binding of their cognate ligands. The interaction of hormone bound estrogen receptors with estrogen responsive elements (EREs) alters transcription of ERE-containing genes. The carboxy terminal region of the estrogen receptor contains the ligand binding domain, the amino terminus serves as the transactivation domain and the DNA binding domain is centrally located. Two forms of estrogen receptor have been identified, ER alpha and ER beta. Both have been shown to be differentially activated by various ligands. The biological response to progesterone is mediated by two distinct forms of the human progesterone receptor (hPR-A and hPR-B) which arise from alternative splicing. In most cells, hPR-B functions as a transcriptional activator of progesterone-responsive gene, whereas hPR-A functions as a transcriptional inhibitor of all steroid hormone receptors.