Even if the diagnosis of HUS and a potential role for complement dysregulation should be evoked early in front of TMA features, TTP (blood ADAMTS13 activity), cobalamin C-related HUS (homocysteinemia) and Shiga-toxin HUS (specific stool or rectal swab culture and shigatoxin PCR) should be first excluded in patients presenting with TMA features. The gene discussed is ADAMTS13; the disease is thrombotic thrombocytopenic purpura.