PROJECT SUMMARY The uterus has the unique ability to support the growth, development, and eventual delivery of offspring. The non-pregnant uterus is no less remarkable: the uterine lining (endometrium) undergoes repeated cycles of shedding during menstruation and subsequent repair, ultimately regenerating approximately 400 times over the reproductive lifespan. This repeated, scarless regenerative process holds immense potential for the identification of new strategies to replace old or damaged tissues, which is a major goal for regenerative medicine. Moreover, understanding endometrial regeneration has important clinical implications, as excessive or insufficient endometrial regeneration gives rise to pathologies that affect the lives of hundreds of millions of women, non- binary people, and transgender men around the world. It is critical to address the longstanding unmet needs of people with endometriosis, adenomyosis, infertility, and to combat the rising incidence of endometrial cancers. The goal of this project is to identify the molecular and cellular basis for regeneration of the menstruating endometrium. A major challenge for the field is that humans belong to a very small group of mammals that menstruate. Thus, although animal models propel many scientific studies, common animal models such as mice and rats have limited utility for studies of menstruation. This project uses the common spiny mouse, which is the only known menstruating rodent, to perform functional tests of the requirements for menstruation. In parallel, we will develop approaches to determine how the human endometrium changes over time. Using these approaches, we aim to understand how the endometrium changes when menstruation begins during puberty and restarts after pregnancy. The proposed studies are an ideal fit for the New Innovator program because our understanding of the menstruating endometrium has lagged far behind more commonly studied regenerative organs. Thus, we must take strategic risks to propel us towards more precise mechanistic understanding of menstruation, a fascinating process that affects the lives of large proportions of the population.