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Yolk sac tumor of the endometrium is an uncommon neoplasm. Here we report a case of yolk sac tumor arising in a uterine carcinosarcoma, with the carcinomatous component showing both endometrioid and serous components, and the sarcomatous component showing homologous (spindled) differentiation. The yolk sac tumor showed predominant glandular configuration and was present admixed with the epithelial components. Extensive immunostaining was performed to narrow the differential diagnosis, including potentially therapeutic testing for HER-2. To our knowledge, this is the first case of carcinosarcoma with this mix of epithelial components and corresponding reporting of these immune and therapeutic markers.