Carcinosarcoma or sarcomatoid carcinoma may be regarded as an epithelial tumor originally in which a subclonal population has undergone a sarcomatoid transformation (or metaplasia): the common cytokeratin- and EMA-positivity of both the epithelial and sarcomatoid parts in our cases and other published cases may support this statement. The gene discussed is MUC1; the disease is carcinosarcoma.