CD99 antigen (Cluster of differentiation 99) also known as MIC2 or single-chain type-1 glycoprotein is a protein that in humans is encoded by the CD99 gene. The protein has a mass of 32 kD. Although present on the X chromosome, unusually, the CD99 gene does not undergo X inactivation, and it was the first such pseudo-autosomal gene to be discovered in humans.
Read more about CD99: Expression