KRT5 and neoplasm: Figure 3 shows staining on fresh tumor (a), tumor after extraction (b), and rat tissue (c). As expected, rat tissue did not stain with HLA Class I. Figure 4 shows staining for cytokeratin (CK) 5/6, confirming SCC. All tumor nests from both fresh and grafted SCC stained diffusely positive for CK5/6. Rat tissue shows some nonspecific CK5/6 staining in the basal layer, (Figure 4C).