AbstractLoss of heterozygosity and promoter hypermethylation ofAPCis frequently observed in human endometrial cancer, which is the most common gynecological cancer in the USA, but its carcinogenic driver status in the endometrial epithelium has not been confirmed. We have identified a novel population of progenitor endometrial epithelial cells (EECs) in mice that express lysozyme M (LysM) and give rise to approximately 15% of all EECs in adult mice. LysM is a glycoside hydrolase that is encoded byLyz2and functions to protect cells from bacteria as part of the innate immune system. Its expression has been shown in a subset of hematopoietic stem cells and in specialized lung and small intestinal epithelial cells. Conditional deletion ofApcin LysM + EECs results in significantly more epithelial cells compared to wild-type mice. At 5 months of age, theApccKOmice have enlarged uterine horns with pathology that is consistent with endometrial hyperplasia with cystic endometrial glands, non-villous luminal papillae and nuclear atypia. Nuclear accumulation of β-catenin and ERα, both of which are known to induce endometrial hyperplasia, was observed in the EECs of theApccKOmice. These results confirm that loss of APC in EECs can result in a phenotype similar to endometrial hyperplasia.