Inclusion Criteria:  Diagnosis of DMD based on clinical phenotype with increased serum creatine kinase. Documented mutation in the DMD gene associated with DMD that is amenable to exon 53 intervention Score of ≥1 on item 1 or 2 of the shoulder component of the Performance of the Upper Limb (PUL).  Stable pulmonary and cardiac function, as measured by the following:  Reproducible percent predicted forced vital capacity (FVC) ≥50%; Left ventricular ejection fraction (LVEF) >55% in patients <10 years of age and >45% in patients ≥10 years of age, as measured (and documented) by echocardiogram (ECHO) or cardiac magnetic resonance imaging (MRI), within 6 months prior to enrollment into the study. Adequate deltoid muscle at Screening to perform open muscle biopsies. Currently on a stable corticosteroid therapy regimen, defined as initiation of systemic corticosteroid therapy that occurred ≥6 months prior to Screening and no changes in dose ≤3 months prior to Screening visit.  Exclusion Criteria:  Cardiac insufficiency:  Severe cardiomyopathy that, in the opinion of the Investigator, prohibits participation in this study; however, cardiomyopathy that is managed by angiotensin-converting-enzyme (ACE) inhibitors or beta blockers is acceptable provided the participant meets the LVEF inclusion criterion. Any other evidence of clinically significant structural or functional heart abnormality. A cardiac troponin I value >0.2 ng/mL on initial and repeat testing if initial test is elevated at Screening. Need for daytime mechanical or noninvasive ventilation OR anticipated need for daytime mechanical or noninvasive ventilation within the next year in the opinion of the Investigator. Nighttime noninvasive ventilation is permitted. Received prior treatment with an investigational peptide-conjugated phosphorodiamidate morpholino oligomer (PPMO) or drisapersen. Received prior treatment with gene therapy for DMD. Received treatment with ataluren, viltolarsen, eteplirsen, or golodirsen within the 14 weeks prior to Screening. Received any investigational drug within 3 months or 5 half-lives, whichever is longer prior to Screening.