Breast cancer is the most common cancer afflicting women in the United States and a leading cause of cancer related deaths. In cases of breast cancer, estrogen receptor status is used clinically both as a prognostic marker of the biological behavior of a tumor and as a guide in therapy. Estrogen receptor is present in high levels in about one half of all breast cancers. Of these two thirds will respond to anti-estrogens or other hormonal manipulations, while less than 10% of the estrogen receptor negative breast cancers will respond. The factors controlling the expression of estrogen receptor are unknown. Over the past few years the reagents needed to dissect the regulation of estrogen receptor at a molecular level have been generated. These include monoclonal antibodies to estrogen receptor, the full length cDNA clone and the putative promoter region. This proposal addresses the question of the regulation of estrogen receptor expression at the molecular level. The important cis-acting elements present in the estrogen receptor gene that are responsible for the tissue specific and "tumor specific" expression of estrogen receptor will be identified using the now standard approach of fusing nested deletions of the promoter region to an easily assayed indicator gene. Following this the specific trans-acting factors will be identified, purified, and cloned using techniques developed or refined to a large extent in the laboratory of the sponsor, Dr. Phillip Sharp. These include the gel electrophoresis DNA binding assay, in vitro transcription, and expression cloning of sequence specific DNA binding proteins. It is hoped that the detailed understanding of the regulation of estrogen receptor expression in molecular terms will contribute to the development of new approaches in the prevention and treatment of breast cancer.