Many factors modulate the proteolysis of VWF by ADA-MTS13. These ligands that bind the A1 domain of VWF such as platelet glycoprotein Iba, heparin and ristocetin promote VWF proteolysis by ADAMTS13 [10] . In addition, platelets also significantly enhance the cleavage of VWF multimers by ADAMTS13 under fluid shear stress [11, 12] . On the contrary, thrombospondin-1, an extracellular matrix adhesion protein, may compete with ADAMTS13 for binding to the A3 domain of VWF, which reduces the rate of VWF proteolysis by ADAMTS13 [13, 14] .