Primary TMA syndromes include TMA whose etiology is known: thrombotic thrombocytopenic purpura (TTP), due to deficiency of the von Willebrand factor-cleaving protease ADAMTS13 (7); typical hemolytic uremic syndrome (HUS), caused by Shiga toxin-producing Escherichia coli (8); pneumococcal-associated HUS (9, 10); and atypical HUS (aHUS) caused by inherited or acquired abnormalities in complement proteins leading to unregulated activation of the alternative pathway of the complement system and the formation of the membrane attack complex (MAC) (11, 12). The gene discussed is ADAMTS13; the disease is thrombotic thrombocytopenic purpura.