CD99
Description
The CD99 (CD99 molecule (Xg blood group)) is a protein-coding gene located on chromosome X|Y.
CD99 antigen (Cluster of differentiation 99), also known as MIC2 or single-chain type-1 glycoprotein, is a heavily O-glycosylated transmembrane protein encoded by the CD99 gene in humans. The protein has a mass of 32 kD. Unusually for a gene present on the X chromosome, the CD99 gene does not undergo X inactivation, and it was the first such pseudoautosomal gene to be discovered in humans. It is expressed on all leukocytes but highest on thymocytes and is believed to augment T-cell adhesion and apoptosis of double positive T cells. It has been found in endothelial cells and in the periodontium, including gingival fibroblasts and gingival epithelial cells. It also participates in migration and activation. There is also experimental evidence that it binds to cyclophilin A. It is found on the cell surface of Ewing's sarcoma tumors and is positive in granulosa cell tumors. It is more expressed in malignant gliomas than in the brain, and such overexpression results in higher levels of invasiveness and lower rates of survival. Antibodies to CD99 are used in diagnostic immunohistochemistry to distinguish Ewing's sarcoma from other tumours of similar histological appearance, as well as for the identification of thymic tumours, and of spindle cell tumours, such as synovial sarcoma, haemangiopericytoma, and meningioma. EWS/FLI is thought to regulate CD99, but knockdown of EWS/FLI results in only a modest reduction in CD99. When CD99 expression is knocked down in human cells with Ewing's sarcoma and those cells are grafted onto mice, tumor and bone metastasis development is reduced.
CD99 is also known as HBA71, MIC2, MIC2X, MIC2Y, MSK5X.
