This study is designed to generate additional data on the effect of ataluren for producing dystrophin protein in nonsense mutation nmDMD participants. This study will evaluate dystrophin levels from participants with nmDMD who currently have been receiving ataluren for ≥9 months.

The study will have a single visit (Visit 1).

Inclusion Criteria:

• Evidence of signed and dated informed consent/assent document(s) indicating that the participant (and/or his parent/legal guardian) has been informed of all pertinent aspects of the trial.
• Ambulatory (10 meters walk/run in less than [<] 30 seconds) and functional grade on the Brooke Upper Extremity Scale of a 1 or a 2.
• Currently being treated with ataluren 10, 10, 20 mg/kg for >=9 months, with no gap in treatment of greater than (>) 1 month, in an ongoing PTC-sponsored nmDMD clinical trial prior to study entry.
• Phenotypic evidence of duchenne muscular dystrophy (DMD) based on the onset of characteristic clinical symptoms or signs (for example, proximal muscle weakness, waddling gait, and Gowers' maneuver) by 6 years of age and an elevated serum creatine kinase (CK). Medical documentation of phenotypic evidence of DMD needs to be provided upon request by the medical monitor.
• Willing to undergo muscle biopsy.

Exclusion Criteria:

• Known contra-indication to muscle biopsy (such as bleeding or clotting disorders).
• Exposure to another investigational drug within 2 months prior to study enrollment or ongoing participation in any non-ataluren interventional clinical trial.
• Requirement for daytime ventilator assistance or any use of invasive mechanical ventilation via tracheostomy. Note: Evening non-invasive mechanical ventilation such as use of bilevel positive airway pressure (Bi-PAP) therapy is allowed.
• Prior or ongoing medical condition (for example, concomitant illness, psychiatric condition, behavioral disorder), medical history, physical findings or laboratory abnormality that, in the investigator's opinion, could adversely affect the safety of the participant, makes it unlikely that the course of treatment or follow-up would be completed, or could impair the assessment of study results.

Intent-to-treat (ITT) population included all enrolled participants with a valid assessment of dystrophin level, as measured by electrochemiluminescence (ECL).

Baseline (Day 1) up to Week 1

Safety population included all participants who received at least 1 dose of ataluren.