VWF and thrombotic thrombocytopenic purpura: Adamts13 encodes a large circulating protease responsible for processing multimeric von Willebrand factor (vWF) in vivo [22, 23]; mutations in human ADAMTS13 cause thrombotic thrombocytopenic purpura [24] and variations in ADAMTS13 activity are associated with other thrombotic abnormalities [25, 26].