Plays a role in a late step of leukocyte extravasation helping cells to overcome the endothelial basement membrane. Acts at the same site as, but independently of, PECAM1 (By similarity). Homophilic adhesion molecule, but these interactions may not be required for cell aggregation (By similarity). Expressed predominantly in the ventral medullary surface of the brain, moderate expression in the cerebral cortex and cerebellum. Low expression in lung and kidney. No expression in heart, stomach, intestine and skeletal muscle. O-glycosylated. Belongs to the CD99 family.