KRT20 and Merkel cell skin cancer: Immunohistochemistry aims to differentiate MCCs from other, morphologically similar, neuroendocrine tumours through characteristic staining of a variety of MCC epithelial and neuroendocrine markers, such as cytokeratin-20 (CK20), neurofilaments, CAM 5.2, TTF-1 and AE1/3 [73,74].