The primary objective of this study is to evaluate acute liver injury (ALI) rates associated with ELEVIDYS with the addition of sirolimus as an adjunct prophylactic immunosuppression agent.

Key Inclusion Criteria:

1. Cohort 1 only: Is male at birth, ambulatory, and ≥ 4 years of age at the time of dosing.
2. Cohort 1 only: Is eligible for commercial ELEVIDYS.
3. Cohort 2 only: Is male at birth and has previously received ELEVIDYS in a commercial setting after pre-treatment with sirolimus and corticosteroids.
4. Cohort 1 only: Participants who are sexually active must agree to use, for the entire duration of the study, a condom and the female sexual partner must also use a medically acceptable form of birth control (eg, oral contraceptive).
5. Has (a) parent(s) or legal guardian(s) who is (are) able to understand and comply with the study visit schedule and all other protocol requirements, or is ≥ 18 years of age and personally able to understand and comply with the protocol requirements.
6. Either has a parent or legal guardian who is willing to provide informed consent, or is ≥ 18 years of age and able to provide informed consent independently.

Key Exclusion Criteria:

1. Cohort 1 only: Contraindicated to receive ELEVIDYS per the United States Package Insert (USPI).

2. Cohort 1 only: Has serological evidence of current, chronic, or active human immunodeficiency virus, hepatitis C, or hepatitis B infection.

3. Has a medical condition or confounding circumstances (eg, prior traumatic limitation for mobility or significant behavioral comorbidity) that, in the opinion of the Investigator, might compromise:

   1. The participant's ability to comply with the protocol-required procedures, and/or
   2. The participant's well-being or safety, and/or
   3. The clinical interpretability of the data collected from the participant

4. Cohort 1 only: Has a symptomatic infection (eg, upper respiratory tract infection, pneumonia, pyelonephritis, meningitis) within 4 weeks prior to Day 1.

5. Cohort 1 only: Has received a live virus vaccine within 4 weeks or inactive vaccine within 2 weeks of the Day 1 visit or expects to receive a vaccination during the first 3 months after Day 1.

6. Cohort 1 only: Any confounding factors that would prevent the use of oral sirolimus including a known hypersensitivity to sirolimus or any of its excipients.

7. Cohort 1 only: Any wounds or recent injuries that, in the opinion of the Investigator, would be at risk of dehiscence or impaired healing in the setting of sirolimus administration.

Other inclusion/exclusion criteria may apply, per protocol.