Involved in T-cell adhesion processes and in spontaneous rosette formation with erythrocytes. Plays a role in a late step of leukocyte extravasation helping leukocytes to overcome the endothelial basement membrane. Acts at the same site as, but independently of, PECAM1. Involved in T-cell adhesion processes (By similarity). Extensively O-glycosylated. The gene coding for this protein is located in the pseudoautosomal region 1 (PAR1) of X and Y chromosomes. Belongs to the CD99 family.