Which immunohistochemical marker is characteristically positive in melanoma?

S100 protein is a well-known immunohistochemical marker that is characteristically positive in melanoma. The significance of S100 lies in its role as a calcium-binding protein that is expressed in various cell types, particularly in neural crest-derived cells, which include melanocytes. In the context of melanoma, S100 serves as a reliable marker due to its sensitivity in detecting malignant melanocytes.

Moreover, the presence of S100 in melanoma can help differentiate it from other neoplasms, making it a valuable tool in the diagnostic process. The sensitivity of S100 for melanoma is notable, as it can be present in both benign and malignant melanocytic lesions, providing a broader context for assessment.

In comparison, the other markers listed do not have the same specificity or diagnostic utility for melanoma. CD20, primarily a marker for B lymphocytes, is not relevant to melanoma specifically. Ki-67 is a marker of proliferation and while it can be expressed in many tumors, including melanoma, it is not specific to it and is more often used to assess the growth fraction of tumor cells rather than as a definitive marker. Cytokeratin, associated with epithelial cells, is not characteristically positive in melanoma, which originates from melanocytes rather than keratinocytes