– Glycoprotein VI (GPVI) is a 63 kDa platelet/megakaryocyte-specific type I transmembrane glycoprotein of the immunoglobulin superfamily that is an important collagen receptor and initiator of platelet activation, aggregation and thrombin generation. GPVI is also a secondary receptor required for platelet spreading on laminin. GPVI associates with the Fc receptor gamma -chain via charged aa in the TM domains of GPVI (arginine) and the FcR gamma (aspartic acid). Collagen binding by the GPVI Ig-like domains initiates signaling through the FcR gamma ITAM sequence. Dimerization of GPVI (2:2 with FcR gamma ) and N-glycosylation greatly enhances collagen binding. Type I and III collagens are strong thrombus-forming components in the vascular subendothelium and atherosclerotic plaques. GPVI initiates binding to fibrillar collagens under flow conditions, then activates integrin alpha 2 beta 1 which binds collagen more tightly. GPVI deficiencies cause only a mild bleeding tendency, probably because integrin alpha 2 beta 1 is able to minimally initiate collagen binding. Normal human GPVI concentration can vary widely and affect maximum thrombin generation. Engagement of GPVI by collagens or other agonists, including autoantibodies, causes calmodulin-regulated metalloproteinase cleavage of the 57 kDa ECD and depletes surface GPVI.