Inclusion Criteria
* Participant must be male at birth, 3 to \<5 years of age, inclusive (Part 1) and 2 to \<3 years of age (Part 2), at the time of legally authorized representative (LAR) signing and dating the informed consent form.
* Ambulatory -as defined as the ability to walk at least 10 meters unassisted (ie, without personal assistance or use of any assistive devices) Note: children who have not yet developed the ability to walk by the time of screening (for whatever reason) will not be eligible for the study.
* Has a definitive diagnosis of DMD prior to Screening or as part of Screening based on genetic testing. Note that participants who rescreen do not have to repeat genetic testing for the diagnosis of DMD if one is already on file. Genetic reports must describe a frameshift deletion, frameshift duplication, premature stop ("nonsense"), canonical splice site mutation, or other pathogenic variant in the DMD gene fully contained between exons 18 to 58 (inclusive) that is expected to lead to absence of a functional dystrophin protein (mutations in exons 1-17 or 59-71 are therefore not permitted).
* Able to cooperate with motor assessment testing.
* Has received vaccinations recommended for the participant's age and DMD disease according to Centers for Disease Control and Prevention (CDC) Child and Adolescent Immunization Schedule by Age, World Health Organization, or local recommendation incorporating the Advisory Committee on Immunization Practices (ACIP) Vaccine Recommendations and Guidelines for Patients with Altered Immunocompetence.
Exception is made for seasonal influenza and coronavirus disease 2019 (COVID-19) vaccines, for which shared decision-making with the participant's physician is encouraged.
Exclusion Criteria
* Prior treatment with gene or cell-based therapy at any time.
* Oligonucleotide-based exon skipping or small molecule stop codon readthrough-promoting therapies for at least 6 months prior to enrolment.
* Has left ventricular ejection fraction \< 50% on the screening echocardiogram (ECHO) or clinical signs and/or symptoms of cardiomyopathy.
* Has cardiac arrhythmia or significant electrocardiogram (ECG) interval abnormalities.
* Major surgery within 3 months prior to Day 1 or planned surgery or procedures that would interfere with the conduct of the study at any time during this study.
* The presence of any other clinically significant illness, including cardiac, pulmonary, hepatic, renal, hematologic, immunologic/allergic, behavioural disease, infection, unhealed injury, malignancy, concomitant illness, extenuating circumstance, or requirement for chronic drug treatment that, in the opinion of the Investigator:
1. Creates unnecessary risks for undergoing gene transfer;
2. Might compromise the participant's ability to comply with the protocol-required testing or procedures; or
3. Might compromise the participant's well-being, safety, or clinical interpretability.
* Has serological evidence of current, chronic, or active human immunodeficiency virus, hepatitis C, or hepatitis B infection.
* Has signs of clinically significant symptomatic infection (eg, upper respiratory tract infection, pneumonia, pyelonephritis, meningitis) within 4 weeks prior to Day 1.
* Has contraindications for IT administration of the product or for lumbar puncture, such as anatomical abnormalities, bleeding disorders or other medical conditions (eg, spina bifida, meningitis, or significant clotting abnormalities).
* Demonstrates cognitive or developmental delay or impairment that could confound assessment of motor development in the opinion of the Investigator.
* Total serum anti-AAV9 antibody titers of \> 1:50 as determined by ELISA within 14 days of Day 1.
Note: Other inclusion/exclusion criteria may apply.
