PROJECT SUMMARY A genetic single-cell lineage tracing system invented by PI Shiying Jin in the mouse uterus has been applied to reveal that epithelial stem cells support endometrial cyclical regeneration during the estrous cycle as well as after parturition. This data supports the idea that stem cells are critically important to maintain uterine homeostasis and function. The objective here is to determine the epithelial stem cell regulation of pregnancy establishment. The main function of the mammalian uterus is to provide an optimal environment for embryo implantation and development. Implantation is a finely orchestrated process that involves interaction between a receptive uterus and competent embryo. Uterine receptivity requires that the endometrial epithelial cells proliferate and then differentiate in response to E2 and P4 from the ovary. Here, we hypothesize that endometrial epithelial stem cells are critical in pregnancy establishment. The research strategy employed in this application is conceptually and technically innovative. The proposed studies will fill a substantial gap in our understanding of the biology of uterine epithelial stem cells, and successful completion is expected to accelerate the development of personalized medicine approaches to treat infertility associated disorders of the endometrium and diseases that impact the reproductive health of women.