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Microphthalmia transcription factor (Mitf), a melanocytic nuclear protein critical for the embryonic development and postnatal viability of melanocytes, is a master lineage regulator and modulates extracellular signals. Recently, Mitf expression was shown to be both a sensitive and specific marker of epithelioid melanoma. Because loss of specific melanocytic markers in melanomas with spindle cell morphology is more common compared with those tumors with epithelioid morphology, we investigated the sensitivity of D5, an anti-Mitf antibody, for diagnosis in this diagnostically problematic subset of melanomas. Twenty of 21 (95%) spindle cell and desmoplastic melanomas examined were reactive for S-100 protein. Only 4 of 21 (19%) spindle cell and desmoplastic melanomas were reactive for HMB-45. Six of 21 tumors (29%) were reactive for D5, including one case that was non-reactive for S-100 and HMB-45. Melan-A reactivity was seen in 2 of 13 cases (15%) studied. Eight of 24 (33%) non-melanocytic spindle cell tumors were reactive for D5, including 4 of 6 dermatofibromas, 1 of 6 schwannomas, 1 of 2 leiomyomas, and 2 of 6 leiomyosarcomas. Although D5 was shown in a previous study to be a highly sensitive and specific marker for epithelioid melanomas, the results of this study show it is not a sensitive or specific marker of spindle cell and desmoplastic melanomas. Nevertheless, we believe that diffuse positive staining for D5 when taken in clinical, histologic and immunohistochemical context may be diagnostically useful in selected cases of melanoma.