What this trial measures

• Change in Height z-scoreAt Enrollment Visit (baseline), and at each Yearly Follow-up Visit (for up to 5 years).

  Standing height in participants 2 years of age and older will be measured using a stadiometer mounted at a right angle between a level floor and against a straight, vertical surface. When transitioning from recumbent length to standing height measurements in participants between 2 to 3 years of age, measure both length and height. If collecting standing height measurements is not feasible, height will be estimated using arm span measurements. To estimate the height, the arm span will be measured from fingertip to fingertip with arms fully extended horizontally. The same standardized technique will be used at all study visits.

• Change in BMI z-scoreAt Enrollment Visit (baseline), and each Yearly Follow-up Visit (for up to 5 years)

  Weight will be measured according to the site's standard of care procedures. For ambulatory patients, this typically involves a standing scale; for non-ambulatory patients, a wheelchair or bed scale may be used. Sites should document the method used at each visit.

• Change in North Star Ambulatory Assessment from baselineAt Enrollment visit, and each Yearly Follow-up Visit (for up to 5 years).

  The North Star Ambulatory Assessment is a clinical assessment scale specifically designed to measure functional ability in ambulant male patients with DMD. The North Star Ambulatory Assessment consists of 17 scored items and 2 timed tests, including the Time to Run/Walk Test and the Time to Stand Test. The Time to Run/Walk Test measures the time (in seconds) that it takes the patient to run or walk 10 meters. The Time to Stand Test measures the time (in seconds) required for the patient to stand in an erect position from supine (floor). Patients should be barefoot and comfortably clothed. North Star Ambulatory Assessment score range is 0 to 34, with higher scores indicating better ambulatory function.

• Change in Performance of Upper Limb from baselineAt Enrollment Visit, at each Yearly Follow-up Visit (for up to 5 years).

  The Performance of Upper Limb assessment is an observer-rated measure of upper-limb function in individuals with Duchenne muscular dystrophy. It includes an entry item to determine the participant's starting functional level and a series of tasks evaluating shoulder, mid-level (elbow), and distal (wrist/hand) abilities. Total scores reflect the participant's ability to perform defined functional movements, with higher scores indicating better function.

• Tanner stage documentationAt Enrollment Visit (baseline), at each Yearly Follow-up Visit (for up to 5 years).

  Pubertal development will be assessed by a physician using Tanner Stages assessment. This assessment uses Male External Genitalia Scale. Stage 1: Testicular volume \< 4 ml or long axis \< 2.5 cm Stage 2: 4 ml-8 ml (or 2.5 to 3.3 cm long), 1st pubertal sign in males Stage 3: 9 ml-12 ml (or 3.4 to 4.0 cm long) Stage 4: 15-20 ml (or 4.1 to 4.5 cm long) Stage 5: \> 20 ml (or \> 4.5 cm long) Pubic Hair Scale Stage 1: No hair Stage 2: Downy hair Stage 3: Scant terminal hair Stage 4: Terminal hair that fills the entire triangle overlying the pubic region Stage 5: Terminal hair that extends beyond the inguinal crease onto the thigh Pubertal development assessments will be discontinued when the patient has completed puberty.

• Percentage of patients with cataract and/or glaucomaAt Enrollment (baseline), and during each Yearly Follow-up Visit (for up to 5 years).

  Presence of cataract and glaucoma will be assessed by an optometrist or ophthalmologist.

• Findings from lateral spine x-rayAt Enrollment Visit (baseline), and during each Yearly Follow-up Visit (for up to 5 years).

  Lateral spine x-ray will be performed to evaluate vertebral fractures and for spine deformities.

• Findings from anterior-posterior/posterior-anterior X-rayAt Enrollment Visit (baseline), and at each Yearly Follow-Up Visit (for up to 5 years).

  Anterior-posterior/Posterior-anterior spine x-ray will be performed to evaluate spine deformities.

• Findings from hand-wrist X-rayAt Enrollment Visit (baseline), and each Yearly Follow-Up Visit (for up to 5 years).

  Posteroanterior hand x-ray will be performed for the bone age assessment.

• Summary statistics of bone mineral density measures assessed by dual x-ray absorptiometryAt Enrollment Visit (baseline) and each Yearly Follow-Up Visit (for up to 5 years).

  Bone mineral content, bone mineral density, bone area, and bone mineral density Z-score will be assesses using dual x-ray absorptiometry (DXA). Summary statistics will be tabulated by anatomical location at each visit.

• Body Composition Mass MeasuresAt Enrollment Visit, each Yearly Follow-Up and End of Study/Early Termination (for approximately 5 years).

  Fat mass, lean mass, and fat-free mass will be assessed using DXA. Summary statistics will be tabulated by visit.

• Summary statistics of body fat percentage and regional fat distribution ratios assessed by DXAAt Enrollment visit, each Yearly Follow-up visit, and End of study/Early termination (for approximately 5 years).

  Body fat percentage, tissue fat percentage, android/gynoid body fat percent ratio, and android/gynoid tissue fat percent ratio will be assessed using DXA. Summary statistics will be tabulated by visit.

• Findings from standard of care echocardiographyAt Enrollment Visit, and at each Yearly Follow-up Visit (for up to 5 years).

  If available as per standard of care, findings from echocardiography will be collected. This is not required if not available as part of the standard of care.

• Frequency and percentage of participants experiencing any adverse events and serious adverse eventsFrom informed consent, Enrollment visit, each Yearly follow-up visit, and End of Study/Early Termination (for up to 5 years).

  Adverse events (AEs) and serious adverse events (SAEs) will be collected after signing the informed consent through the end of study participation. The frequency and percentage of patients experiencing any AE or SAE will be summarized. Adverse events will be assessed and documented by the investigator based on routine clinical evaluations, including vital signs, physical examinations, laboratory assessments, and medical record review. For each adverse event, the Investigator will document relevant clinical details, including timing, severity, outcome, and assessments of the seriousness and causality. Adverse events will be assessed by the current version of the CTCAE at the time the adverse event is identified, and will be summarized by MedDRA system organ class and preferred term using the current version of MedDRA.

• Findings from cardiac magnetic resonance imaging (subset of patients taking part in the cardiac sub-study only)Enrollment Visit, and at each Yearly Follow-Up Visit (for up to 5 years).

  Myocardial damage will be assessed through magnetic resonance imaging using late gadolinium enhancement. Late gadolinium enhancement is a technique used in cardiac MRI for cardiac tissue characterization, in particular, the assessment of myocardial scar formation and regional myocardial fibrosis

• Presence of cardiomyopathy assessed by echocardiography using transmural strain profile (only in a subset of patients taking part in the cardiac sub-study)At Enrollment Visit (baseline), and each Yearly Follow-up Visit (for up to 5 years) or Early Termination.

  Presence of cardiomyopathy will be assessed by echocardiography using transmural strain profile (only in a subset of patients taking part in the cardiac sub-study).