Duchenne/Becker muscular dystrophy (DBMD) is a genetic disorder that develops in boys. It is caused by a mutation in the gene for dystrophin, a protein that is important for maintaining normal muscle structure and function. Loss of dystrophin causes muscle fragility that leads to weakness and loss of walking ability during childhood and teenage years. A specific type of mutation, called a nonsense (premature stop codon) mutation, is the cause of DBMD in approximately 10-15% of boys with the disease. Ataluren is an orally delivered, investigational drug that has the potential to overcome the effects of the nonsense mutation. This study is an open-label trial for patients with nonsense mutation dystrophinopathy who received ataluren in a prior PTC-sponsored study at a US clinical trial site. This trial will be conducted at sites in the US and will evaluate the long-term safety of ataluren, as determined by adverse events and laboratory abnormalities. Patients will receive study drug 3 times per day (at breakfast, lunch, and dinner). Study assessments will be performed at clinic visits during screening and every 12 weeks until the end of the study.