CD63 belongs to the tetraspanin family, which is characterized by four transmembrane domains, one short extracellular domain (ECL1), and one long extracellular domain (ECL2). Tetraspanins interact with a variety of cell surface proteins and intracellular signaling molecules in specialized tetraspanin-enriched microdomains (TEMs) where they mediate a range of processes, including adhesion, motility, membrane organization, and signal transduction. CD63, like other tetraspanins, is enriched in exosomes. It is also a component of Weibel-Palade bodies found in endothelial cells. Research studies demonstrate several functions of CD63 in different cell types, including roles in mast cell degranulation, VEGF signaling in endothelial cells, recruitment of leukocytes to endothelial cells, and endosomal sorting during melanogenesis.