• 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.