The purpose of this study was to evaluate the efficacy and safety of ivacaftor in subjects with cystic fibrosis aged 6 to 11 years who have the G551D mutation in the cystic fibrosis transmembrane conductance regulator (CFTR) gene. Ivacaftor is a potent and selective potentiator of wild-type, G551D, F508del, and R117H forms of human CFTR protein. Potentiators are pharmacological agents that increase the chloride ion transport properties of the channel in the presence of cyclic adenosine monophosphate (AMP)-dependent protein kinase A (PKA) activation.