A Study to Assess the Safety, Tolerability, and Pharmacokinetics of BIIB078 in Adults With C9ORF72-Associated Amyotrophic Lateral Sclerosis

Key Inclusion Criteria: * Ability of the participant to understand the purpose and risks of the study, to provide signed and dated informed consent, and to authorize the use of confidential health information in accordance with national and local participant privacy regulations; or, in the event of the participant's physical incapacity to sign, to confirm that understanding and consent orally to a legally authorized representative (LAR) for the express purpose of having said informed consent and authorization signed on his/her behalf. * All participants of childbearing potential must agree to practice highly effective contraception during the study and be willing and able to continue contraception for 5 months after their last dose of study treatment. * Must meet the possible, laboratory-supported probable, probable, or definite criteria for diagnosing ALS according to the World Federation of Neurology El Escorial criteria and have documentation of a clinical genetic test demonstrating the presence of a pathogenic mutation in C9ORF72. * Slow vital capacity (SVC) ≥ 50% of predicted value as adjusted for sex, age, and height (from the sitting position). * Participants taking concomitant riluzole at study entry must be on a stable dose for ≥ 30 days prior to the first dose of study treatment (Day 1). * Participants taking concomitant edaravone at study entry must be on a stable dose for ≥ 60 days prior to the first dose of study treatment (Day 1). * ALS Cognitive Behavioral Scr…