Inclusion Criteria:

• Ability to provide written informed consent (parental/guardian consent if applicable)/assent (if <18 years of age)
• Male sex.
• Age ≥5 years.
• Phenotypic evidence of DMD/BMD based on the onset of characteristic clinical symptoms or signs (ie., proximal muscle weakness, waddling gait, and Gowers' maneuver) by 9 years of age, an elevated serum creatinine kinase level, and ongoing difficulty with walking.
• Documentation of the presence of a nonsense point mutation in the dystrophin gene as determined by gene sequencing from a laboratory certified by the College of American Pathologists (CAP), the Clinical Laboratory Improvement Act/Amendment (CLIA) or an equivalent organization.
• Ability to walk ≥75 meters unassisted during the screening 6-minute walk test.
• Documentation that a baseline renal ultrasound has been performed.
• Confirmed screening laboratory values within the central laboratory ranges (adrenal, renal, and serum electrolytes parameters)
• Willingness and ability to comply with scheduled visits, drug administration plan, study procedures, laboratory tests, and study restrictions.

Exclusion Criteria:

• Treatment with systemic aminoglycoside antibiotics within 3 months prior to start of study treatment.
• Initiation of systemic corticosteroid therapy within 6 months prior to start of study treatment or change in systemic corticosteroid therapy (eg, initiation, change in type of drug, dose modification not related to body weight change, schedule modification, interruption, discontinuation, or reinitiation) within 3 months prior to start of study treatment.
• Any change (initiation, change in type of drug, dose modification, schedule modification, interruption, discontinuation, or reinitiation) in prophylaxis/treatment for congestive heart failure within 3 months prior to start of study treatment.
• Treatment with warfarin within 1 month prior to start of study treatment.
• Prior therapy with PTC124.
• Known hypersensitivity to any of the ingredients or excipients of the study drug (Litesse® UltraTM [refined polydextrose], polyethylene glycol 3350, Lutrol® micro F127 [poloxamer 407], mannitol 25C, crospovidone XL10, hydroxyethyl cellulose, vanilla, Cab-O-Sil® M5P [colloidal silica], magnesium stearate).
• Exposure to another investigational drug within 2 months prior to start of study treatment.
• History of major surgical procedure within 30 days prior to start of study treatment.
• Ongoing immunosuppressive therapy (other than corticosteroids).
• Ongoing participation in any other therapeutic clinical trial.
• Expectation of major surgical procedure (eg, scoliosis surgery) during the 12 month treatment period of the study.
• Requirement for daytime ventilator assistance.
• Clinical symptoms and signs of congestive heart failure (American College of Cardiology/American Heart Association Stage C or Stage D) or evidence on echocardiogram of clinically significant myopathy
• Prior or ongoing medical condition (eg, concomitant illness, psychiatric condition, behavioral disorder, alcoholism, drug abuse), medical history, physical findings, electrocardiogram findings, or laboratory abnormality that, in the investigator's opinion, could adversely affect the safety of the subject, makes it unlikely that the course of treatment or follow-up would be completed, or could impair the assessment of study results.