Key Inclusion Criteria: * Ambulatory participants, defined as able to walk without assistive aid, 10MWR \<30 seconds, and NSAD total score ≥25; non-ambulatory participant, defined as 10MWR ≥30 seconds or unable to perform, and PUL 2.0 entry scale score ≥3. * Ambulatory participants must be 4 to 20 years of age and the non-ambulatory participant must be ≥4 years of age. * All participants must be ≤70 kilograms * Possess 1 homozygous or 2 heterozygous pathogenic and/or likely pathogenic α-SG deoxyribonucleic acid (DNA) gene mutations as documented prior to screening. * Able to cooperate with muscle testing. * Participants must have adeno-associated virus (AAV) serotype Rh74 (rh74) antibody titers \<1:400 (that is, not elevated) as determined by an enzyme-linked immunosorbent assay (ELISA). Key Exclusion Criteria: * Left ventricular ejection fraction \<40% or clinical signs and/or symptoms of cardiomyopathy * FVC ≤40% of predicted value and/or requirement for nocturnal ventilation * Any other clinically significant illness, including neuromuscular (other than limb girdle muscular dystrophy type 2D/R3 \[LGMD2D/R3\]), that in the opinion of the Investigator might compromise the participant's ability to comply with the protocol required testing or procedures or compromise the participant's wellbeing, safety, or clinical interpretability. Other inclusion/exclusion criteria apply.