CD99 is a type I single chain transmembrane protein devoid of N-linked glycosylation sites encoded by the pseudoautosomal gene MIC2. CD99 has an apparent molecular weight of 32 kD and is widely expressed on a variety of tissues. CD99 is highly expressed on thymocytes, T cells, and T cell leukemias and lymphomas. However, it is absent on some B cell lines, fetal B cells, eosinophils, granulocytes and the NK-cell line YT. CD99 is involved in spontaneous rosette formation with erythrocytes and may also be involved in other T-cell and hematopoietic cell adhesion pathways. CD99 has been reported to activate a caspase-independent death pathway in T cells under some conditions. CD99 interacts with a number of proteins including ferritin heavy chain 1, karyopherin beta 1, TRIP13, cyclophilin A, annexin II, and ubiquitin-conjugating enzyme E2H.