CD63, also known as LIMP, LAMP-3, gp55, and melanoma-associated antigen (ME491), is a member of the tetraspanin superfamily (TM4SF) that constitutes a main component of the lysosomal membrane. It is expressed on activated platelets, monocyte/macrophages, endothelium, fibroblasts, osteoblasts, and smooth muscle cells. CD63 may be involved in platelet activation and is thought to function as a transmembrane adaptor protein. CD63 has been shown to associate with CD9, CD81, VLA-3, and VLA-6. In mice, there are two CD63 gene loci, of which only one is functional. CD63 deficient mice are viable, and there is no alteration in the population of immune cells. Recent report shows that CD63-deficient mice exhibit a significant reduction in both leukocyte rolling and recruitment in a peritonitis model.