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 spinal muscular atrophy (SMA) Type 1 and who are genetically defined by a biallelic pathogenic mutation of the survival motor neuron 1 gene (SMN1) with one or two copies of survival motor neuron 2 gene (SMN2). The primary objective of the study is to evaluate the efficacy of onasemnogene abeparvovec-xioi by assessing the proportion of symptomatic SMA Type 1 participants who achieve the ability to sit unaided for at least 10 seconds up to and including the 18 months of age trial visit. At least 6 participants aged \< 6 months (\< 180 days) at the time of gene replacement therapy (Day 1) will be enrolled.