The protein encoded by this gene is a cell surface glycoprotein involved in leukocyte migration, T-cell adhesion, ganglioside GM1 and transmembrane protein transport, and T-cell death by a caspase-independent pathway. In addition, the encoded protein may have the ability to rearrange the actin cytoskeleton and may also act as an oncosuppressor in osteosarcoma. Cyclophilin A binds to CD99 and may act as a signaling regulator of CD99. This gene is found in the pseudoautosomal region of chromosomes X and Y and escapes X-chromosome inactivation. Two transcript variants encoding different isoforms have been found for this gene.