Safety, Tolerability, Pharmacodynamic, Efficacy, and Pharmacokinetic Study of DYNE-251 in Participants With Duchenne Muscular Dystrophy Amenable to Exon 51 Skipping

Inclusion Criteria: * Age 4 to 16 years inclusive, at the time of informed consent/assent. * Male with a confirmed diagnosis of DMD and with a mutation in the dystrophin gene characterized by exon deletion amenable to exon 51 skipping. * Upper extremity muscle group that is amenable to muscle biopsy. * Brooke Upper Extremity Scale score of 1 or 2. * Ambulatory or non-ambulatory. A non-ambulatory participant must have been non-ambulatory for \<2 years before enrollment. * Receiving a stable dosage of glucocorticoids for at least 12 weeks prior to the start of study drug administration, with the expectation of maintaining a stable dose during the Placebo-Controlled and Open-Label Periods of the study (unless dose adjustment is required by weight change). * Left ventricular ejection fraction of ≥50% by echocardiogram or ≥55% by cardiac magnetic resonance imaging (MRI). Exclusion Criteria: * Uncontrolled clinical symptoms and signs of congestive heart failure (CHF). * Any change in prophylaxis/treatment for CHF within 3 months prior to the start of study treatment. * History of major surgical procedure within 12 weeks prior to the start of study drug administration or an expectation of a major surgical procedure during the study. * Requirement of daytime ventilator assistance. * Percent predicted FVC \<40 % (applies only for participants who are age ≥7 years). * Receipt of eteplirsen, or alternative exon-skipping/dystrophin-modifying therapy, within 12 weeks of randomization. * Receipt of non-exon skipping investigational drug within 4 months before the start of study drug administration. * Receipt of gene therapy at any time. Other inclusion and exclusion criteria may apply.