CD63 is a 238 amino acid, tetraspanin glycoprotein of ~40-60 kDa. CD63 is expressed by monocytes, macrophages, endothelial cells where it is a component of Weibel-Palade bodies and by a variety of cell lines. It is located intracellularly in lysosomal granules of platelets, being translocated to the surface upon activation. It may therefore be useful as a marker of platelet activation. CD63 expression is also associated with melanoma development and expression is rapidly down-regulated by progesterone in endometrial tissues.
