Malignant melanoma is commonly known as the great mimicker and can present in various clinical manifestations and with diverse morphological variants. One of the rare histological variants is the signet-ring cell type. The signet-ring morphology has been reported in numerous other neoplasms including adenocarcinoma, lymphoma, liposarcoma, squamous cell carcinoma, and basal cell carcinoma. We report a rare case of primary signet-ring cell malignant melanoma in a 62-year-old man. He initially presented with an enlarging nevus on his right flank with surrounding erythema. A biopsy showed atypical epithelioid and signet cells with prominent nucleoli and occasional mitoses. Initial diagnosis favored metastatic signet-cell carcinoma of gastrointestinal origin. Review of the biopsy and immunohistochemical analysis revealed the malignant signet-ring cells stained with S100, vimentin, and melanoma cocktail, in keeping with melanoma. The signet-cell morphology can be found in a variety of other malignancies. To prevent potential misdiagnoses, thorough histological examination should be aided by an appropriate immunohistochemical panel to confirm melanoma and exclude erroneous differentials.
