A Study to Learn About Salanersen's (BIIB115) Effects on Movement and Its Safety When Given Before Symptoms Appear in Babies With Genetically Diagnosed Spinal Muscular Atrophy (SMA)

For Part 1 of the study, the primary objective is to evaluate the clinical efficacy of salanersen in participants with genetically diagnosed SMA, and the secondary objective is to evaluate safety and tolerability, pharmacokinetics (PK), and effect on biomarkers after treatment with salanersen in participants with genetically diagnosed SMA. For Part 2 of the study, the primary objective is to evaluate the long-term clinical efficacy of salanersen in participants with genetically diagnosed SMA, and the secondary objective is to evaluate the long-term safety and tolerability, PK, and effect on biomarkers after treatment with salanersen in participants with genetically diagnosed SMA.