In this study, we demonstrate that onasemnogene abeparvovec, administered during the first 6 weeks post-partum to infants with biallelic SMN1 mutations and two SMN2 copies, but no clinical signs of SMA, alters the natural course of disease and results in better motor outcomes, ventilator-free survival, and nutritional and respiratory independence as compared with untreated patients with SMA type 1 or those treated after symptom onset. Here, SMN2 is linked to proximal spinal muscular atrophy.