Complement factor I (CFI) mutations are implicated in the pathogenesis of atypical hemolytic uremic syndrome (aHUS). Nevertheless, there is evidence that CFI deficiency is a weak effector of aHUS. Bienaime et al. report that homozygous deletion of CFHR-1 in the RCA gene cluster of chromosome 1q is a major risk factor for poor outcome for patients with CFI mutations. The basic and clinical implications of the findings are further elaborated here.
