Mutations in complement proteins predispose to atypical hemolytic uremic syndrome (aHUS). Mutation screening in aHUS is challenging, because most of the disease-associated mutations are individually rare, and a significant proportion of variants consist of missense mutations of unknown significance. The definitive interpretation of a variant of unknown significance (VUS) is often dependent on a reliable functional assay too time-consuming to be used in a diagnostic screening service. Allied research groups have analyzed these VUSs in aHUS.