This entry represents the extracellular domain or large extracellular loop (LEL) of CD63. This extracellular domain lies between the 3rd and 4th trans-membrane segment. CD63 is present in platelets, neutrophils, and endothelial cells, amongst others. In platelets it associates with the integrin alphaIIBbeta3 and may modulate alphaIIBbeta3-dependent cytoskeletal reorganization [, ].