VWF and thrombotic thrombocytopenic purpura: In CASA/Rk mice with homozygous ADAMTS13 null alleles, spontaneous thrombosis and death occur in post-neonatal life.107 Only administration of shiga toxin is able to induce a massive secretion of UL-VWF multimers from endothelial cells.109 From a clinical standpoint, there is no evidence of antecedent shiga toxin exposure in patients of TTP.