Cluster of differentiation 99 (CD99), also known as MIC2 or T-cell surface glycoprotein E2, is a glycosylated transmembrane protein expressed by lymphocytes, cortical thymocytes, granulosa cells of the ovary, pancreatic islet cells, Sertoli cells, and endothelial cells. CD99 is a type I, single chain transmembrane protein devoid of N-linked glycosylation sites that is encoded by the pseudoautosomal gene MIC2. CD99 has an apparent molecular weight 32 kD and is widely expressed on a variety of tissues. It is involved in T-cell adhesion, leukocyte migration and differentiation of primitive neuroectodermal cell. CD99 produces diffuse membrane staining patterns on nearly all Ewing's sarcoma and primitive peripheral neuroectodermal tumors. CD99 may be found in synovial sarcoma, neuroendocrine carcinoma, acute myeloid leukemia, mesenchymal chondrosarcoma, lymphoblastic lymphoma, small round blue cell tumors, solitary fibrous tumors, vascular tumors, and myeloid sarcoma and produce heterogeneous staining patterns, which must be accompanied by other antibody staining for a final diagnosis.