CD99 is a MIC2 gene product and is found in lymphocytes, thymic cortical cells, granulosa cells of the ovary, pancreatic islet cells and supporting cells of the testis. CD99, as detected with a variety of antibodies, is expressed by virtually almost all Ewings sarcoma and primitive peripheral neuroectodermal tumors (ES/PNET) and demonstrates strong and diffuse membranous staining.^1-4 Other tumors that may show CD99 expression include neuroendocrine carcinomas, mesenchymal chondrosarcomas, solitary fibrous tumors, synovial sarcomas, vascular tumors, small round blue cell tumors, lymphoblastic lymphoma, acute myeloid leukemia, and myeloid sarcoma.5 However, strong and diffuse membranous reactivity for CD99 favors ES/PNET over the other diagnostic considerations.5 The other CD99+ tumors usually show cytoplasmic and more heterogeneous staining. Therefore, when making a final diagnostic interpretation, CD99 must be considered in a panel with other antibodies. ^4-5