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 subject 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 subject privacy regulations; or, in
the event of the subject'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 subjects 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).

- Subjects 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).

- Subjects 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 Screen (ALS-CBS) score ≥ 11 for the cognitive portion; ≥ 33
for the behavioral portion.

- 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.

- Screening values of coagulation parameters including platelet count, international
normalized ratio (INR), prothrombin time (PT), and activated partial thromboplastin
time (APTT) should be within normal ranges.

- Has an informant/caregiver who, in the Investigator's judgment, has frequent and
sufficient contact with the subject as to be able to provide accurate information
about the subject's cognitive and functional abilities at Screening.

Key Exclusion Criteria:

- History of drug abuse or alcoholism ≤ 6 months of Screening that would limit
participation in the study, as determined by the Investigator.

- Tracheostomy.

- History of or positive test result at Screening for human immunodeficiency virus. .

- History of, or positive test result at Screening for, hepatitis C virus antibody.

- Treatment with another investigational drug or biological agent within 1 month of
Screening or 5 half-lives of study agent, whichever is longer.

- Treatment with anti-platelet or anticoagulant therapy ≤ 14 days before Screening (with
the exception of aspirin ≤ 325 mg/day) or anticipated use during the study.

- Current or anticipated need, in the opinion of the Investigator, of a diaphragm pacing
system during the study period.

- Female subjects who are pregnant or currently breastfeeding.

NOTE: Other protocol defined Inclusion/Exclusion criteria may apply.