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 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 participant as to be able to provide accurate information
   about the participant'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.

   - Prescreening ALSFRS-R slope less than 0.4 points/month, where prescreening ALSFRS-R
   slope is defined as follows: (48 - ALSFRS-R score at Screening) / (months from date of
   symptom onset to date of Screening).

   - 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 an antiplatelet or anticoagulant therapy that cannot safely be
   interrupted for lumbar puncture (LP) according to local standard of care and/or
   institutional guidelines, in the opinion of the Investigator or Prescriber.

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

   - Female participants who are pregnant or currently breastfeeding.

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