Mutations in the complement factor H (CFH) gene are frequently associated with atypical hemolytic uremic syndrome (aHUS). Hakobyan et al. have developed novel reagents that can rapidly determine the contribution of each CFH allele to the total plasma CFH pool, showing that low-expression CFH alleles are important risk factors for the development of aHUS. These reagents represent a significant contribution to the techniques used to determine susceptibility factors among individuals with aHUS.