CD63 is a 53 kD type III lysosomal glycoprotein also known as LIMP, LAMP-3, gp55, and melanoma-associated antigen (ME491). CD63 is a member of the tetraspan transmembrane superfamily (TM4SF) protein expressed on activated platelets, monocytes/macrophages, endothelium, fibroblasts, osteoclasts, 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.