This Phase 3 study will evaluate the efficacy of ION363 on clinical function and survival in carriers of fused in sarcoma mutations with amyotrophic lateral sclerosis (FUS-ALS). ION363 is based on antisense technology, which is a therapy for reducing disease-causing proteins before they are created. The investigational antisense medicine (called ION363) is designed to target and stop the FUS gene from making its protein, which builds up in motor neurons and may contribute to symptoms of ALS. The investigational antisense medicine is administered by an intrathecal injection (an injection into the fluid surrounding your spinal cord).