PBGENE-DMDPBGENE-DMD Phase 1/2a Safety and Preliminary Efficacy Study in Duchenne Muscular Dystrophy (FUNCTION-DMD)

Inclusion Criteria:

1. Males, 2 to 7 years of age, inclusive, at the time of informed consent/assent
2. Molecular confirmed DMD diagnosis (DMD mutation fully contained between exons 45 to 55 \[inclusive\])
3. Clinical phenotype consistent with DMD in the opinion of the Investigator
4. Ability to complete age-appropriate motor testing assessments requirements.

Participants aged 2 to < 4 years at the time of screening must:
1. Be able to walk at least 10 meters independently (without assistive devices).
2. Be able to rise from the floor without physical assistance (use of a Gowers' maneuver is acceptable).

Participants aged 4 to 7 years at the time of screening must:
3. Be able to walk at least 100 meters independently (without assistive devices).
4. Have an NSAA total score between 16 and 29, inclusive.
5. Participant has received age-appropriate routine childhood immunizations per the local country's national immunization schedule.
6. The participant's parent(s)/LAR(s) are willing and able to provide written informed consent prior to the initiation of any trial-specific procedures; where applicable, the participant must provide written or verbal assent in accordance with local regulations.
7. The participant and their parent(s)/LAR(s) are willing to participate in a LTFU study after the completion of this trial.

Exclusion Criteria:

1. Prior treatment with any gene therapy, gene editing therapy, or cell-based therapy at any time.
2. Receipt of any investigational medication or experimental therapy within 6 months prior to Day 1.
3. Prior or ongoing use of any product designed to increase dystrophin expression, investigational, or otherwise, including exon-skipping therapies, within 6 months of the scheduled Day 1 dose or inability or unwillingness to refrain from initiating or resuming these therapies for at least 5 years following gene therapy administration.
4. Prior ongoing use of any product designed to increase dystrophin expression, investigational, or otherwise, including exon-skipping therapies, within 6 months of the scheduled Day 1 dose.
5. Concurrent enrollment in another clinical trial, unless it is observational (non-interventional).
6. A positive test for antibodies to AAV9
7. A participant has any condition that would contraindicate treatment with immunosuppression.
8. Participants with pathogenic mutations in exons 1-44 and/or exons 56-79.
9. Evidence of cardiomyopathy or clinically significant left ventricular dysfunction, defined as LVEF <50% on screening echocardiogram.