Unknown risk factor(s) to atypical hemolytic uremic syndrome can be associated with identified mutations: example from three families. In the 3 families, one child with aHUS has a mutation in CFH or CFI while a sibling also with aHUS has no mutation identified. Therefore the two siblings in each family share at least one unidentified risk factor. Affected individuals are indicated by filled symbols. Carriers of mutations are indicated by an asterisk. Courtesy of Professors R. Salomon (Hôpital Necker, Paris), E. Bérard (Hôpital de l'Archet, Nice) and G. Deschênes (Hôpital Robert Debré, Paris), with permission.