Eligible patients with nonsense-mutation cystic fibrosis ( aged 6 years ; abnormal nasal potential difference ; sweat chloride > 40 mmol/L ; forced expiratory volume in 1 s [ FEV1 ] 40 % and 90 % ) were randomly assigned by interactive response technology to receive oral ataluren ( 10 mg/kg in morning , 10 mg/kg midday , and 20 mg/kg in evening ) or matching placebo for 48 weeks .