A white male at risk for SMA based on family history (one older sister who became symptomatic at 14 months of age and treated with nusinersen and another older sister diagnosed at 10 days of age and treated with nusinersen) and the presence of only three copies of SMN2 was treated with intravenous onasemnogene abeparvovec at 2 months of age prior to symptom onset. This evidence concerns the gene SMN2 and proximal spinal muscular atrophy.