KRT20 and basal cell carcinoma: Therefore, immunohistochemistry helps in distinguishing between MCC and other skin tumors such as basal cell carcinoma (CK20−), lymphoma (CD45+), small cell melanoma (S100+ CK20−), or metastatic neuroendocrine carcinoma (CD56+, synaptophysin+ TTF-1+, CK20−) [42,43].