An Extension Study to Assess the Long-Term Safety, Tolerability, Pharmacokinetics, and Effect on Disease Progression of BIIB067 Administered to Previously Treated Adults With Amyotrophic Lateral Sclerosis Caused by Superoxide Dismutase 1 Mutation

– Must have diagnosis of SOD1-ALS, and must have completed the End of Study Visit for
either Parts A, B, or C of Study 233AS101 (NCT02623699) (i.e., were not withdrawn).

– If taking riluzole, must be receiving a stable dose for ≥30 days prior to Day 1.

– For female participants of childbearing potential must agree to practice effective
contraception during the study and be willing and able to continue contraception for 5
months after their last dose of study treatment.

– 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.

– Participants from Study 233AS101 Parts A and B must have a washout ≥16 weeks between
the last dose of study treatment received in Study 233AS101 and the first dose of
BIIB067 received in the current Study 233AS102.

– If taking edaravone, participant must have initiated edaravone ≥60 days (2 treatment
cycles) prior to Day 1. Edaravone may not be administered on dosing days during this
study.

Key Exclusion Criteria:

– History of allergies to a broad range of anesthetics.

– Presence of risk for increased or uncontrolled bleeding and/or risk of bleeding that
is not managed optimally and could place a participant at an increased risk for
bleeding during or after a Lumbar Puncture (LP) procedure. These risks could include,
but are not limited to, anatomical factors at or near the LP site (e.g., vascular
abnormalities, neoplasms, or other abnormalities) and underlying disorders of the
coagulation cascade, platelet function, or platelet count (e.g., hemophilia, Von
Willebrand’s disease, liver disease).

– Presence of an implanted shunt for the drainage of CSF or an implanted central nervous
system (CNS) catheter.

– Prior or current treatment with small interfering ribonucleic acid (RNA), stem cell
therapy, or gene therapy.

– Treatment with another investigational drug, biological agent (excluding BIIB067), or
device within 1 month or 5 half-lives of study agent, whichever is longer.

– Current or anticipated need, in the opinion of the Investigator, of a diaphragm pacing
system (DPS) during the study period.

– Female participants who are pregnant or currently breastfeeding.

– 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.

NOTE: Other protocol defined Inclusion/Exclusion criteria may apply