KRT20 and Merkel cell skin cancer: The tumor cells of MCC show a characteristic neuroendocrine cytomorphology with scant cytoplasm and uniform round to oval nuclei., show in the majority of cases, positivity for cytokeratin 20 (CK20), synaptophysin, chromogranin A (CHGA), and neurofilament (NF), but often negative for thyroid transcription factor 1 (TTF-1) [6].