For instance, inactivation of the ADAMTS13 gene in mice failed to generate the phenotype of TTP microvascular thrombosis until the ADAMTS13 null allele was transferred to a particular mouse strain, CASA/Rk, that has increased levels of VWF.107,108 Nevertheless, cross-breeding studies showed that the development of TTP is independent of mouse plasma VWF levels. This evidence concerns the gene VWF and thrombotic thrombocytopenic purpura.