This study is a randomized, double-blind, placebo-controlled, 72-week study followed by a 72-week open-label period. The purpose is to characterize the long-term effects of ataluren-mediated dystrophin restoration on disease progression. Patients will be randomized in a 1:1 ratio to ataluren or placebo. Patients will receive blinded study drug TID at morning, midday, and evening for 72 weeks, after which all patients will receive open-label ataluren for an additional 72 weeks (144 weeks in total). Study assessments will be performed at clinic visits every 12 weeks during the double-blind period and every 24 weeks during the open-label period. The total sample size of ~250 subjects will include ~160 subjects who meet the criteria for inclusion in the primary analysis population (age 7 to 16 years old, baseline 6MWD >=300 meters, supine to stand >=5 seconds). The study will be conducted in the United States and other countries around the world.