The estrogen receptor (ER) is a 66 kDa protein that mediates the actions of estrogens in estrogen responsive tissues. It is a member of a large superfamily of nuclear hormone receptors that function as ligand activated transcription factors. The ER gene consists of more than 140 kb of genomic DNA divided into 8 exons. These translate into a protein with six functionally discrete domains, labeled A through F. A second form of the estrogen receptor, ER beta has recently been described. The ER is an important regulator of growth and differentiation in the mammary gland. Presence of ER in breast tumors indicates an increased likelihood of response to anti estrogen (e.g. tamoxifen) therapy.