Summary of Work: The relatively high incidence of carcinomas that develop in the breast and uterus of human females has prompted a strong interest to understand the causative factors in these diseases. The concern for estradiol in the etiology of endometrial cancer originated from the finding about thirty years ago that postmenopausal females treated with this ovarian steroid develop a significant increase in this cancer. The influence of estradiol on breast cancer risk is less clear, but there is compelling evidence that ovarian steroids have a permissive role in the spontaneous disease. On this basis we sought to identify growth regulating pathways that are influenced by ovarian steroids. In one experimental model, we screened for receptor tyrosine kinases and their substrates in the mouse uterus that exhibit increased tyrosine phosphorylation in response to estradiol. Using this approach, we identified tyrosine phosphorylated forms of the insulin-like growth factor-1 receptor (IGF-1R) and the docking protein insulin receptor substrate-1 (IRS-1). Immunoprecipitates of uterine IRS-1 from estrogen- treated animals contained phosphoinositide 3-kinase (PI3-K), which may be important in conferring a mitogenic signal in uterine epithelial cells. Formation of the IRS-1/PI3-K complex probably originates from estrogen- induced ligand activation of IGF-1R kinase since transgenic mice (IGF-1 m/m) with an impaired capacity to synthesize IGF-1 form negligible amounts of tyrosine phosphorylated IRS-1 in response to estradiol. Thus, the methodologies used allow detection of signaling components in cells of whole animals that are influenced by steroid hormones. We have currently extended this experimental approach to identifying hormone- responsive growth factor pathways in the human uterus, developing mammary gland of the mouse, and autonomous growth regulatory signaling pathways in uterine and breast neoplasms.