CD63, also known as melanoma-associated antigen ME491, is a member of the tetraspanin superfamily, which is characterized by proteins containing four transmembrane domains (1). CD63 is extensively N-glycosylated and is predominantly localized to late endosomes, lysosomes, secretory vesicles, and the plasma membrane (1–3). It is ubiquitously expressed in normal tissues and is markedly upregulated in activated basophils and mast cells, where it is required for efficient IgE-mediated degranulation (4,5).
In melanoma, CD63 is highly expressed during early stages of disease, where it has been implicated in the suppression of epithelial-to-mesenchymal transition (EMT); however, its expression is reduced in more advanced lesions (6). In addition, CD63 expression has been reported to possess prognostic significance in several other solid tumors (7).