An open-label, single-arm, ascending-dose clinical trial assessed the safety and efficacy of onasemnogene abeparvovec-xioi in subjects < 2 years old with genetically confirmed bi-allelic SMN1 gene deletions, two copies of the SMN2 gene, and absence of the c.859G > C modification in exon 7 of the SMN2 gene, and with SMA symptom-onset before 6 months of age. Here, SMN1 is linked to proximal spinal muscular atrophy.