S100 protein is a sensitive marker for melanomas and peripheral nerve sheath tumors. It is, however, expressed by other mesenchymal and epithelial tumors. Despite its low specificity, S100 protein is valuable for the diagnosis of desmoplastic melanomas and peripheral nerve sheath tumors, for which no specific marker is available. Sox10 is a neural crest transcription factor crucial for specification, maturation, and maintenance of Schwann cells and melanocytes. Anti-Sox10 antibody was applied to a variety of neural crest-derived tumors, mesenchymal and epithelial neoplasms, and normal tissues. Sox10 nuclear expression was found in 76 of 78 melanomas (97%) and 38 of 77 malignant peripheral nerve sheath tumors (49%) whereas S100 protein was expressed in 71 melanomas (91%) and 23 malignant peripheral nerve sheath tumors (30%). Sox10 was diffusely expressed in schwannomas and neurofibromas. Sox10 reaction was seen only in sustentacular cells of pheochromocytomas/paragangliomas, and occasionally carcinoid tumors from various organs, but it was not seen in the tumor cells. In normal tissues, Sox10 was expressed in Schwann cells, melanocytes, and myoepithelial cells of salivary, bronchial, and mammary glands. Sox10 reaction was not identified in any other mesenchymal and epithelial tumors except for myoepitheliomas and diffuse astrocytomas. Sox10 was expressed by metastatic melanomas and nodal capsular nevus in sentinel lymph nodes, but not by other lymph node components such as dendritic cells. Our results indicate that Sox10 will serve as a more sensitive and specific marker for the diagnosis of melanocytic and schwannian tumors than S100 protein.