CD99 [SEP] chromosome
CD99
CD99 antigen (Cluster of differentiation 99), also known as MIC2 or single-chain type-1 glycoprotein, is a heavily O-glycosylated transmembrane protein that is 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.
Expression.
It is expressed on all leukocytes but highest on thymocytes and is believed
result of a translocation between chromosomes 11 and 22, which fuses the EWS gene of chromosome 22 to the FLI1 gene of chromosome 11.
EWS/FLI functions as the master regulator. Other translocations are at t(21;22) and t(7;22). Ewing's sarcoma cells are positive for CD99 and MIC2, and negative for CD45.
Diagnosis.
The definitive diagnosis is based on histomorphologic findings, immunohistochemistry and molecular pathology.
Ewing's sarcoma is a small-blue-round-cell tumor that typically has a clear cytoplasm