VWF and thrombotic thrombocytopenic purpura: However, recent advances have demonstrated that UL-VWF-platelet thrombi in TTP result from the deficiency of a VWF cleavage protease which is now known as ADAMTS-13 (a disintegrin and metalloproteinase with a thrombospondin type 1 motif, member 13), whereas microangiopathy and thrombosis as in HUS result mainly from exposure to Shiga toxins or abnormalities in regulation of the complement system.4