A neoplastic clonal expansion of endometrial glands characterized by cytologic changes of the epithelium and the presence of an increased number of endometrial glands. The glands form crowded aggregates with tubular or branching patterns which are cytologically distinct from the background architectural and cytological pattern. It is associated with molecular changes seen in endometrioid endometrial carcinoma, including microsatellite instability, PAX2 inactivation, and PTEN, KRAS, and CTNNB1 gene mutations. [from
NCI]