 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.