Phase III testing targeted ataluren—which stimulates production of dystrophin protein—as a treatment for Duchenne muscular dystrophy (DMD), which is caused by inadequate stores of the protein. The study pool included male patients aged 7–16 years with nonsense mutation DMD and a 6-minute walk distance (6MWD) of at least 150 meters, who were evenly randomized to ataluren or placebo. At followup at week 48, change in 6MWD from baseline—an indicator of expected disease progression over 1 year—was measured in the intent-to-treat population of 228 boys. Although the drug was well tolerated, with the few adverse events that did occur mild to moderate in nature, it did not produce an overall meaningful improvement in 6MWD compared with placebo. The exception occurred in the prespecified subgroup of patients with a baseline 6MWD of 300–400 meters, who experienced a more predictable rate of decline over 1 year. The researchers for the multisite, multinational ACT DMD study believe their findings can inform future research that uses 6MWD as an outcome.