Duchenne muscular dystrophy (DMD) 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 DMD 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 a Phase 2a extension trial that will evaluate the long-term safety of ataluren in boys with nonsense mutation DMD, as determined by adverse events and laboratory abnormalities. The study will also assess changes in walking, muscle function, strength, and other important clinical and laboratory measures.