Merkel cell carcinoma (MCC) is a rare neuroendocrine tumor that is highly aggressive in nature and indolent in progression. The common risk factors for MCC are senility, prolonged exposure to sunlight, and immune deficient states. Moreover, Merkel cell polyomavirus has recently been characterized to be significantly associated with pathogenesis of MCC, including the expression of Cytokeratin 20 (CK20). Diagnosis is often difficult since histopathological results require a number of differential diagnoses through immunohistochemical (IHC) stains with other cutaneous malignancies. A 67-year-old man presented with a solitary dome-shaped erythematous round mass on the left upper arm for 2 months. Biopsy and IHC studies revealed findings consistent with Merkel Cell Carcinoma of neuroendocrine origin. Common IHC stains usually confirm positive findings for CK20, which is also recognized as the key component in making the diagnosis. We present a CK20 negative MCC in light of expanding the knowledge of unusually stained IHC results in MCC.
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