Here, we present a peculiar case of EC with mixed endometrioid and mucinous features in the form of IM/diff, characterized by MMR defects (MSH2/MSH6 loss of expression and MSI-H); the abovementioned differential diagnoses (metastases, and yolk-sac tumor with intestinal-like features, other variants of mucinous endometrial carcinomas) were excluded after wide clinical, morphological and immunohistochemical work-up. The gene discussed is MSH6; the disease is yolk sac tumor.