TTF1 and primitive neuroectodermal tumor: Specifically, MCC cells stain positively for epithelial markers such as CK20 in a peculiar “dot-like” fashion (negative in extraskeletal PNET/Ewing's sarcoma) and stain negatively for epithelial markers such as LCA (positive in malignant lymphoma), S-100 (positive in malignant melanoma), and CK7 and TTF-1 (positive in small cell lung carcinoma) [17].