CD81 antigen, also known as TAPA1, is a tetraspanin characterized by four transmembrane domains and a member of the transmembrane 4 superfamily. CD81 has broad tissue distribution and may play multiple roles in a variety of cellular processes such as immune response and tumorigenesis, but its function has not been elucidated entirely. It is reported that the tetraspanins CD9, CD63 and CD81 are highly enriched on a certain type of extracellular vesicles, called exosome. Because of various exosome features, CD81, one of the representative exosome surface proteins, is thought to be related to cell-cell communication.