The role of estrogen in the growth of human breast cancers has been investigated at two levels. First, we have studied the pS2 gene, whose transcription is stimulated by estrogen in the human breast cancer cell line, MCF-7. The pS2 gene product is a small, secreted polypeptide currently of unknown function, but with structural features similar to some growth factors. The expression of the pS2 gene has so far been detected only in MCF-7 cells and some breast cancer biopsies. Preliminary studies indicate that pS2 is a potential marker for hormone-dependent breast cancer. Ongoing studies will continue to focus on the implicated role of pS2 in the estrogen-mediated growth of breast cancers and its possible use as a marker for estrogen-dependent tumors. Second, we have analyzed the structure and function of the human ER. The receptor stimulates pS2 gene transcription by interacting with an ERE in the 5'-flanking region of that gene. A mutational analysis of the receptor protein has localized a DNA-binding domain, which determines target gene specificity, and a hormone-binding domain. These domains appear to be the only two regions of the receptor which are absolutely required for the transcription-activating function of the ER in transfection assays with reporter plasmids. The N-terminal region of the protein (regions A and B), which is necessary for increasing the efficiency of gene expression using the pS2 ERE, but not a vitellogenin ERE, may also play a role in transcription activation. Further progress in the characterization of the ER functional domains will require studies on target genes in a more physiological chromatin environment, as well as detailed physical analyses of receptor structure.