Classically, primary TMAs have been classified according to the identification of the following pathogenic mechanisms: thrombotic thrombocytopenic purpura (TTP), mediated by a deficiency in the activity of ADAMTS-13 enzyme; typical hemolytic uremic syndrome, caused by a Shiga-toxin–producing Escherichia coli (STEC-HUS); and primary atypical HUS (aHUS), due to the dysregulation of the alternative complement pathway (ACP) (142). The gene discussed is ADAMTS13; the disease is thrombotic thrombocytopenic purpura.