Single-Dose Gene Replacement Therapy Using for Patients With Spinal Muscular Atrophy Type 1 With One or Two SMN2 Copies

This is a Phase 3, open-label, single-arm, single-dose trial of onasemnogene abeparvovec-xioi (gene replacement therapy) in participants with SMA Type 1 with one or 2 copies of SMN2. At least 6 participants \< 6 months (\< 180 days) of age at the time of gene replacement therapy (Day 1) will be enrolled.

The trial includes 3 trial periods: screening, gene replacement therapy, and follow-up. During the screening period (Days -30 to -2), participants whose parent(s)/legal guardian(s) provide informed consent will undergo screening procedures to determine eligibility for trial enrollment. participants who meet the entry criteria will enter the in-patient gene replacement therapy period (Day -1 to Day 3). On Day -1, participants will be admitted to the hospital for pre-treatment baseline procedures. On Day 1, participants will receive a one-time intravenous (IV) infusion of the equivalent of onasemnogene abeparvovec-xioi cohort 2 dose received in the AVXS-101-CL-101 trial over approximately 60 minutes and will undergo in-patient safety monitoring over the next 48 hours. Participants may be discharged 48 hours after gene replacement therapy, based on Investigator judgment. During the outpatient follow-up period (Days 4 to End of Trial at 18 months of age), participants will return at regularly scheduled intervals for efficacy and safety assessments until the participant reaches 18 months of age.