Sry-related HMg-Box gene 10 (SOX10) is a nuclear transcription factor that plays an important role in melanocytic cell differentiation. It has been shown to be a sensitive marker of melanoma including spindle and desmoplastic subtypes. We assessed its frequency of expression in melanoma, carcinoma, benign nevi, and non-neoplastic tissues with routine immunohistochemistry for SOX10. The 109 primary melanoma included 49 epithelioid, 19 spindle cell, 22 desmoplastic, and 19 mixed spindle cell/desmoplastic melanoma. All primary, except 8 desmoplastic melanoma, and 11 metastatic melanoma were strongly and diffusely nuclear SOX10-positive. Six desmoplastic melanoma had ≤10% cells positive, and 2 were <50% positive, all of 3+ intensity. Eighteen of 149 (12%) breast carcinoma were SOX10-positive. All 24 ovarian, 23 endometrial, 26 lung, and 25 colon carcinoma were SOX10-negative. All 43 benign nevi, 18 dysplastic nevi, 68 non-neoplastic and benign skins, and all 56 non-neoplastic breast tissue were SOX10-positive. The sensitivity and specificity for SOX10 in the diagnosis of melanoma are 1.0 and 0.93, respectively; the positive and negative predictive values are 0.87 and 1.0, respectively. SOX10 is a sensitive, specific marker for melanoma. As benign nevi also express SOX10, it cannot be used to differentiate between benign and malignant pigmented skin lesions. Only a small number of breast carcinoma (12%), and breast lobules, express SOX10; no carcinoma of the ovary, endometrium, lung, or colon expressed SOX10.