In both syndromes, TTP results from a severe deficiency of ADAMTS13, which is responsible for the impaired proteolytic processing of high-molecular-weight von Willebrand factor (HMW-VWF) multimers that, under shear stress of >30 dyn/cm2, are stretched and form long strings that are able to avidly interact with platelets and subendothelial collagen. Here, VWF is linked to thrombotic thrombocytopenic purpura.