Despite the poorly differentiated and/or spindled morphology and the staining of some tumor cells for α-smooth muscle actin (Figure 2D), tumor cells from all but one of the models were immunohistochemically positive for cytokeratin 8 (CK8), confirming that they are epithelial in origin and not mouse fibroblast-derived (Figure 2E and Supplementary Figure 2). The gene discussed is KRT8; the disease is neoplasm.