In recent years, the central role of alternative complement activation in rare inherited forms of genetic or atypical HUS (aHUS) has been established.6 Mutations in both complement inhibitory factors (including CFH, factor I, membrane co-factor protein [MCP / CD46], and thrombomodulin), proteins that promote alternative complement pathway amplification (C3 and factor B) and anti-factor H antibodies have been described.6,7,30 These inherited diseases have constant “low-grade” alternative complement activation and often present insidiously. The gene discussed is C3; the disease is atypical hemolytic-uremic syndrome.