A Study to Assess the Safety, Tolerability, and Effect on Disease Progression of BIIB105 in Participants With Amyotrophic Lateral Sclerosis (ALS) and Participants With the ALS Ataxin-2 (ATXN2) Genetic Mutation

The ALSpire Study is a clinical trial evaluating the investigational drug BIIB105 in adults living with amyotrophic lateral sclerosis (ALS). The ALSpire Study consists of two parts: * Part 1: 6-month placebo-controlled study. During Part 1, participants are randomly assigned to receive either BIIB105 or placebo in a 3:1 or 2:1 ratio (depending on the participant's assigned Cohort). * Part 2: up to 3-year long-term open-label extension. During Part 2, all participants receive BIIB105. The objectives of the study are to evaluate: * The safety and tolerability of BIIB105 in people with ALS * What the body does to BIIB105 (also called "pharmacokinetics") * What BIIB105 does to the body (also called "pharmacodynamics") * Whether BIIB105 can slow the worsening of clinical function