These investigations defined a heritable form of TTP with severe (<5%) ADAMTS13 activity deficiency and an acquired form due to the elaboration of antibodies directed at 1 or more ADAMTS13 epitopes.77 However, many thrombotic microangiopathies (TMAs) are not associated either with severe ADAMTS13 activity deficiency or with antibodies that block ADAMTS13 activity. This evidence concerns the gene ADAMTS13 and Genetic thrombotic microangiopathy.