SMN1 and proximal spinal muscular atrophy: Using this technique, AAV9-mediated SMN gene expression delivered at postnatal day 1 (P1) significantly improved lifespan and motor symptoms in SMA models.32, 33, 34 Initial studies also showed that early treatment of SMA mice at P1 generated better outcomes compared with treatment at P5 or P10, highlighting the importance of early intervention in SMA therapy.34