CD99 antigen is also known as MIC2, single-chain type-1 glycoprotein, HBA71, MIC2X, MIC2Y, MSK5X, 12E7, which belongs to the CD99 family. CD99 / MIC2 is expressed on all leukocytes but highest on thymocytes. Involved in T-cell adhesion processes and in spontaneous rosette formation with erythrocytes. CD99 / MIC2 plays a role in a late step of leukocyte extravasation helping leukocytes to overcome the endothelial basement membrane. CD99 acts at the same site as, but independently of PECAM1. There is also experimental evidence that it binds to cyclophilin A.