A Study to Evaluate the Efficacy, Safety, Tolerability, Pharmacokinetics, and Pharmacodynamics of BIIB067 Administered to Adult Subjects With Amyotrophic Lateral Sclerosis and Confirmed Superoxide Dismutase 1 Mutation

– Weakness attributable to ALS and documented SOD1 mutation at Screening Visit 2.

– A forced vital capacity (FVC) ≥50% of predicted value as adjusted for sex, age, and
height (from the sitting position). Participants with stable FVC <50% but ≥45%, whose FVC has not declined by more than 5% in the last 6 months may be considered for inclusion, at the discretion of the Investigator. - If taking riluzole, participant must be on a stable dose for ≥30 days prior to Day 1 and expected to remain at that dose until the final study visit. - Medically able to undergo the study procedures, and to adhere to the visit schedule at the time of study entry, as determined by the Investigator. Key Exclusion Criteria: Part A and B - History of or positive test result for human immunodeficiency virus. - History of, or positive test result at Screening, for hepatitis C virus antibody. - Current hepatitis B infection (defined as positive for hepatitis B surface antigen [HBsAg] and/or hepatitis B core antibody [HBcAb]). Participants with immunity to hepatitis B from previous natural infection (defined as negative HBsAg, positive hepatitis B surface antibody immunoglobulin G, and positive HBcAb) or vaccination (defined as positive anti-HBs) are eligible to participate in the study. - Treatment with another investigational drug, biological agent, or device within 1 month or 5 half-lives of study agent, whichever is longer. Specifically, no prior treatment with small interfering ribonucleic acid, stem cell therapy, or gene therapy is allowed. - Current enrollment in any other interventional study. - Current or recent (within 1 month) use, or anticipated need, in the opinion of the Investigator, of copper (II) (diacetyl-bis (N4-methylthiosemicarbazone)) or pyrimethamine. - Current or anticipated need, in the opinion of the Investigator, of a diaphragm pacing system (DPS) during the study period. Key Inclusion Criteria: Part C - Weakness attributable to ALS and confirmed SOD1 mutation at Screening Visit. - If taking riluzole, participant must be on a stable dose for ≥30 days prior to Day 1 and expected to remain at that dose until the final study visit. - If taking edaravone, participant must have initiated edaravone ≥60 days (2 treatment cycles) prior to Day 1 and expected to remain at that dose until the final study visit, unless the Investigator determines that edaravone should be discontinued for medical reasons, in which case it may not be restarted during the study. Edaravone may not be administered on dosing days of this study. - Medically able to undergo the study procedures and to adhere to the visit schedule at the time of study entry, as determined by the Investigator. Key Exclusion Criteria: Part C - History of or positive test result for human immunodeficiency virus. - Current hepatitis C infection (defined as positive hepatitis C virus [HCV] antibody and detectable HCV ribonucleic acid [RNA]). Participants with positive HCV antibody and undetectable HCV RNA are eligible to participate in the study (United States Centers for Disease Control and Prevention). - Current hepatitis B infection (defined as positive for HBsAg and/or anti-HBc). participants with immunity to hepatitis B from previous natural infection (defined as negative HBsAg, positive anti-HBc, and positive anti-HBs) or vaccination (defined as negative HBsAg, negative anti-HBc, and positive anti-HBs) are eligible to participate in the study. - Treatment with another investigational drug (including investigational drugs for ALS through compassionate use programs), biological agent, or device within 1 month or 5 half-lives of study agent, whichever is longer. Specifically, no prior treatment with small interfering RNA, stem cell therapy, or gene therapy is allowed. - Current enrollment in any other interventional study. - Current or recent (within 1 month) use, or anticipated need, in the opinion of the Investigator, of copper (II) (diacetyl-bis(N4-methylthiosemicarbazone)) or pyrimethamine. - Current or anticipated need, in the opinion of the Investigator, of a DPS during the study period. NOTE: Other protocol defined Inclusion/Exclusion criteria may apply.