CD63 (LAMP-3, lysosome-associated membrane protein-3), a glycoprotein of tetraspanin family, is present in late endosomes, lysosomes and secretory vesicles of various cell types. It is also present in the plasma membrane, usually following cell activation. Hence, it has become an widely used basophil activation marker. In mast cells, however, CD63 exposition does not need their activation. CD63 interacts with integrins and affects phagocytosis and cell migration, it is also involved in H/K-ATPase trafficking regulation of ROMK1 channels. CD63 also serves as a T-cell costimulation molecule. Expression of CD63 can be used for predicting the prognosis in earlier stages of carcinomas.