The diagnosis of acquired TTP is confirmed by an ADAMTS13 activity level <10% and the presence of ADAMTS13 IgG, while congenital TTP (cTTP) is diagnosed when ADAMTS13 activity level is <10% in the absence of ADAMTS13 IgG and confirmed by mutational analyses (191). The gene discussed is ADAMTS13; the disease is thrombotic thrombocytopenic purpura.