Study of DYNE-251 in Participants with Duchenne Muscular Dystrophy Amenable to Exon 51 Skipping

Prior treatment

• 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/dystrophinmodifying 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.

• 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 enrolment. • Receiving a stable dosage of glucocorticoids for at least 12 weeks prior to the start of study drug administration. • Left ventricular ejection fraction of ≥50% by echocardiogram or ≥ 55% by cardiac magnetic resonance imaging (MRI).