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. Homophilic adhesion molecule, but these interactions may not be required for cell aggregation. Concentrated at cell-cell contacts in cultured endothelial cells. Highly expressed in the nervous system, including brain, dentate nucleus of hippocampus, granular and Purkinje cells of cerebellum, brain stem nucleus and choroid plexus. Expressed in peripheral blood T- and B-cells and neutrophils (at protein level). Almost undetectable in bone marrow-derived neutrophils (at protein level). Also expressed in thymocytes (at protein level) with higher expression in cortical thymocytes than in medullary thymocytes. Expressed at high levels in testis (mostly in germ cells and Sertoli cells) and ovary (mostly in granulosa cells). Expressed in lung, heart, kidney and liver (at protein level); however, expression in heart, kidney and liver seems restricted to endothelial cells (at protein level). Highly expressed in endothelial cells and to a lower level in vascular smooth muscle cells (at protein level). Low expression in spleen. At 12.5 dpc, expressed in most tissues, especially in the nervous system, including the cerebral cortex, cerebellum, spinal cord and ganglion. There is no change in expression pattern from 12.5 dpc to neonatal day 1. Highly expressed in the subventricular zone and cortical plate of fetal brain and in the dorsal root ganglion of the peripheral nervous system. Except in the nervous system, expression in adult tissues is weaker than in fetal ones. O-glycosylated. Seems to be the major transcript. Belongs to the CD99 family.