Merkel cell carcinoma. A, Nests of basophilic tumor cells displaying round nuclei, powdery chromatin, scant cytoplasm, and nuclear molding. B, Tumor cells stained with CK20 antibody show a membranous and dot-like paranuclear pattern. C, Immunohistochemical stain for synaptophysin shows diffuse cytoplasmic positivity. D, In situ hybridization of the Merkel cell polyomavirus RNA shows positive nuclear staining. (A, Hematoxylin-eosin stain; B, CK20; C, synaptophysin; D, Merkel cell polyomavirus RNA ISH; original magnifications: A-C, ×200; D, ×400.)