Duchenne/Becker muscular dystrophy (DMD/BMD) 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/BMD in approximately 10-15% of boys with the disease. PTC124 is an orally delivered, investigational drug that has the potential to overcome the effects of the nonsense mutation. This study is a Phase 2b trial that will evaluate the clinical benefit of PTC124 in boys with DMD/BMD due to a nonsense mutation. The main goals of the study are to understand whether PTC124 can improve walking, activity, muscle function, and strength and whether the drug can safely be given for a long period of time.