These results correlated with the findings reported by theInternational Society on Thrombosis and Hemostasis (ISTH) registry that functional assays of vWF either vWF:RCo or collagen binding activity (vWF:CBA) appeared to be the most sensitive measurement of the vWF defect in acquired von Willebrand disease (vWD) including MPN (Federici et al., 2000). This evidence concerns the gene VWF and von Willebrand disease (hereditary or acquired).