Thrombotic thrombocytopenia purpura (TTP), defined clinically by microangiopathic hemolytic anemia and thrombocytopenia, is attributed to either inherited or acquired loss of VWF multimer size regulation caused by severe ADAMTS13 deficiency, resulting in the accumulation of UL–VWF multimers. This evidence concerns the gene ADAMTS13 and thrombotic thrombocytopenic purpura.