ADAMTS13 and thrombotic thrombocytopenic purpura: The estimated annual incidence of idiopathic TTP is 3.7 to 11 cases per million.36 In extremely rare cases, severe deficiency of ADAMTS13 (defined as <5% serum activity) is related to compound heterozygous or homozygous mutations of the ADAMTS13 gene (Upshaw-Shulman syndrome).37–39 Defects in coding of the metalloprotease gene, located on chromosome 9q34, result in functionally deficient enzyme.