Vimentin is used as an indicator of mesenchymal differentiation; α-SMA and desmin for muscle differentiation; and S-100 protein for nerve and cartilage differentiation.14) Although molecular analysis and vimentin immunostaining, which is commonly positive in the sarcomatous component of carcinosarcoma, were not performed in this case, the overall morphological and immunohistochemical profile (negative for epithelial markers AE1/AE3 and CAM5.2, and focally positive for muscle markers such as α-SMA, desmin, and caldesmon) strongly supported the diagnosis of a sarcomatous component. The gene discussed is ACTA1; the disease is carcinosarcoma.