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)

Key Inclusion Criteria: -≤42 days of age at first dose of salanersen. * Genetic documentation of 5q SMA homozygous gene deletion or mutation or compound heterozygous mutation. * Two or three copies of the survival motor neuron 2 (SMN2) gene. * Ulnar compound muscle action potential (CMAP) amplitude ≥2 millivolt (mV) at Screening and Day 1 predose. * Body weight ≥3rd percentile for age based on World Health Organization (WHO) Child Growth Standards at the time of informed consent. Key Exclusion Criteria: * Any clinical signs or symptoms at Screening or Day 1 predose that are, in the opinion of the Investigator, strongly suggestive of SMA. * Areflexia on neurologic examination at biceps, knee, or ankle at Screening or Day 1 Predose. * Hypoxemia (oxygen saturation \<96% awake or asleep without any supplemental oxygen or respiratory support, or for altitudes \>1000 meters (m), oxygen saturation of \<92% awake or asleep without any supplemental oxygen or respiratory support). * Diagnosis of neonatal respiratory distress syndrome necessitating surfactant replacement therapy or invasive ventilatory support. * Any reason, anatomical or otherwise (including hematology/coagulation laboratory results), that presents increased risk of complication from the LP procedures or safety assessments. * Any prior treatment with an approved SMA disease-modifying therapy (e.g., nusinersen, onasemnogene abeparvovec-xioi \[OA\], and/or risdiplam), a myostatin inhibitor therapy, or an investigational drug given for the treatment of SMA. Note: Other protocol-defined inclusion/exclusion criteria will apply.