Change in Height z-score

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.

At Enrollment Visit (baseline), and at each Yearly Follow-up Visit (for up to 5 years).

Change in BMI z-score

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.

At Enrollment Visit (baseline), and each Yearly Follow-up Visit (for up to 5 years)

Change in North Star Ambulatory Assessment from baseline

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.

At Enrollment visit, and each Yearly Follow-up Visit (for up to 5 years).

Change in Performance of Upper Limb from baseline

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.

At Enrollment Visit, at each Yearly Follow-up Visit (for up to 5 years).

Tanner stage documentation

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.

At Enrollment Visit (baseline), at each Yearly Follow-up Visit (for up to 5 years).

Percentage of patients with cataract and/or glaucoma

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

At Enrollment (baseline), and during each Yearly Follow-up Visit (for up to 5 years).

Findings from lateral spine x-ray

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

At Enrollment Visit (baseline), and during each Yearly Follow-up Visit (for up to 5 years).

Findings from anterior-posterior/posterior-anterior X-ray

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

At Enrollment Visit (baseline), and at each Yearly Follow-Up Visit (for up to 5 years).

Findings from hand-wrist X-ray

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

At Enrollment Visit (baseline), and each Yearly Follow-Up Visit (for up to 5 years).

Summary statistics of bone mineral density measures assessed by dual x-ray absorptiometry

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.

At Enrollment Visit (baseline) and each Yearly Follow-Up Visit (for up to 5 years).

Body Composition Mass Measures

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

At Enrollment Visit, each Yearly Follow-Up and End of Study/Early Termination (for approximately 5 years).

Summary statistics of body fat percentage and regional fat distribution ratios assessed by DXA

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.

At Enrollment visit, each Yearly Follow-up visit, and End of study/Early termination (for approximately 5 years).

Findings from standard of care echocardiography

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.

At Enrollment Visit, and at each Yearly Follow-up Visit (for up to 5 years).

Frequency and percentage of participants experiencing any adverse events and serious adverse events

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.

From informed consent, Enrollment visit, each Yearly follow-up visit, and End of Study/Early Termination (for up to 5 years).

Findings from cardiac magnetic resonance imaging (subset of patients taking part in the cardiac sub-study only)

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

Enrollment Visit, and at each Yearly Follow-Up Visit (for up to 5 years).

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

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).

At Enrollment Visit (baseline), and each Yearly Follow-up Visit (for up to 5 years) or Early Termination.

Changes in Duchenne Muscular Dystrophy Quality of Life questionnaire score

Duchenne Muscular Dystrophy Quality of Life questionnaire is a patient-reported outcome measure designed to assess how Duchenne muscular dystrophy affects physical, emotional, and social aspects of daily life. Duchenne Muscular Dystrophy Quality of Life questionnaire proxy report will be completed by a parent/guardian for patients aged 7 years and older. For patients aged 10 years and older, a self-report will be completed by the patient, and, if feasible, a proxy report will also be completed by the parent or guardian.

At Enrollment Visit (baseline) and at each Yearly Follow-up Visit for up to 5 years.

Change in Patient Reported Outcome Measure for the Upper Limb score

Patient Reported Outcome Measure for the Upper Limb is a questionnaire for young males/children with DMD from the age of 7 years. The PROM UL will be completed by parent/legal guardian up to the age of 16 years and by the patient from the age of 17 years. It consists of 32 items covering four domains of activities of daily living: (1) food, (7 items, max score 14); (2) self-care, (8 items, max score 16); (3) household and environment, (6 items, max score 12); (4) leisure and communication, (11 items, max score 22). For each question, the patients and/or their parent/legal guardian are asked to report their perceived difficulty in performing the activity on a 3-level scale: Cannot do (0); Can do with difficulty (1); Can do easily (2). The maximum total score is 64, with higher score indicating better function.

At Enrollment Visit (baseline), and each Yearly Follow-Up Visit (for up to 5 years).