One-Time gene infusion aims to restore missing muscle protein in children

Copied word for word from the study's registry entry, so the wording is the study team's rather than ours.

Inclusion Criteria: * Participant and/or legal representative understands the purpose, nature, procedures, and potential risks of the study and provides written informed consent. * Age 4 years to \<13 years at the time informed consent is signed; male or female. * Pathogenic compound heterozygous or homozygous variants in SGCA confirmed by genetic testing. * Absent or reduced alpha-sarcoglycan expression confirmed by muscle-biopsy pathology. * Serum creatine kinase (CK) \>1000 U/L with typical clinical signs and/or symptoms of LGMD2D/R3. * Receiving a stable dose of systemic corticosteroid therapy for at least 3 months before screening: prednisone/prednisolone \>=0.75 mg/kg/day, deflazacort \>=0.9 mg/kg/day, or vamorolone oral suspension 6 mg/kg/day (weight-based adjustment allowed), with no significant change in dose/regimen anticipated during the study. * Able to climb 4 stairs without assistance. * Screening anti-BBM-L101 capsid-binding antibody titer \<=1:200 and neutralizing antibody titer \<=1:4. * Able to complete manual muscle testing, motor-function assessments, lower-limb MRI, and muscle biopsy as required by the protocol. * At screening and Day -1: hemoglobin \>=80 g/L; platelets \>=100 x 10\^9/L; GGT \<=1.5 x ULN; total bilirubin \<=1.5 x ULN; and serum creatinine \<1.5 x ULN. * Participant and/or legal representative is willing and able to comply with study procedures and scheduled follow-up. Exclusion Criteria: * Positive hepatitis B surface antigen (HBsAg), HBV DNA \>=1000 U/mL, positive HCV RNA, or positive HIV test. * Currently receiving antiviral therapy for hepatitis B, hepatitis C, or HIV infection. * Severe behavioral or cognitive problems that, in the investigator's judgment, would preclude study participation. * Poorly controlled asthma; clinically significant pulmonary-function decline due to LGMD2D/R3; or recurrent infectious pneumonia judged likely to affect respiratory function. * Left ventricular ejection fraction (LVEF) \<50% or New York Heart Association (NYHA) functional class III or higher. * Severe or persistent arrhythmia (including atrial fibrillation, frequent premature ventricular contractions, ventricular bigeminy/trigeminy, or severe bundle-branch block) or congenital heart disease. * Change in preventive therapy or cardiomyopathy treatment within 1 month before study treatment (initiation, drug-class change, dose change, schedule change, interruption, discontinuation, or restart). * Underlying clinically significant liver disease, including prior portal hypertension, splenomegaly, hepatic encephalopathy, liver fibrosis stage \>=3, or clinically significant nodules/cysts or elevated alpha-fetoprotein at screening. * Use of any systemic immunosuppressant within 30 days before dosing other than investigator-recommended corticosteroids and eculizumab (inhaled or topical steroids are permitted), or known hypersensitivity to corticosteroids, eculizumab, or other planned immunosuppressive agents. * Vaccination within 30 days before dosing or planned vaccination during screening through Week 26 after infusion. * Surgery within 3 months before dosing or planned surgery during the first 52 weeks after infusion. * Prior participation in a gene- or cell-therapy study for LGMD2D/R3; use of another investigational drug or a newly globally approved LGMD2D/R3 therapy within 6 months before dosing; or long-term regular use of traditional Chinese medicine/herbal products for LGMD2D/R3 within 6 months before dosing. * Current or previous autoimmune disease, severe renal or gastrointestinal disease, major psychiatric/neurologic disorder, coagulation disorder, malignancy, or other severe medical condition. * Any other condition that, in the investigator's judgment, makes the participant unsuitable for the study.