Merkel cell carcinoma (MCC). A, High-magnification photomicrograph shows that the tumor is composed of blue cells with round, granular-appearing nuclei and frequent mitoses. B, Immunohistochemistry with cytokeratin 20 shows the characteristic pattern of perinuclear dotlike staining. Molecular studies for Merkel cell polyomavirus (MCPyV) DNA are positive in approximately 80% of MCCs.