ER gene consists of more than 140kb of genomic DNA divided into 8 exons, being translated into a protein with six functionally discrete domains required for transcription activation function, binding to estrogen response element (ERE) constitutive dimerization, binding to heat shock proteins, and ligand recognition. 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.