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.
