The primary objective of the study is to evaluate the pharmacokinetics (PK) of BIIB104 in healthy Japanese and non-Japanese participants. The secondary objective of the study is to evaluate the safety and tolerability of multiple, oral doses of BIIB104 administered twice daily (BID) for 9 days, with an additional dose occurring in the morning on Day 10 in healthy Japanese and non-Japanese participants.

Key Inclusion Criteria:

• Have a body mass index between 18 and 30 kilograms per meter square (kg/m^2), inclusive, and total body weight >50 kilograms (kg) [110 pounds (lb)].
• For Japanese participants, was born in Japan, and biological parents and grandparents were of Japanese origin.
• For Japanese participants, if living outside Japan for more than 5 years, must not have significantly modified diet since leaving Japan.
• Non-Japanese participants must have a screening weight within ±20% of the mean value for Japanese participants.

Key Exclusion Criteria:

• Participation in other studies involving treatment with an investigational drug within 30 days or 5 half-lives (whichever is longer) prior to randomization and/or during study participation.
• History of severe allergic or anaphylactic reactions, systemic hypersensitivity reaction to BIIB104, or any allergic reactions that in the opinion of the investigator are likely to be exacerbated by any component of the study treatment.
• History of seizures or a condition with risk of seizures.
• History of, or positive test result at Screening for, human immunodeficiency virus (HIV).
• Chronic, recurrent, or serious infection, as determined by the investigator, within 6 months prior to screening or between screening and Day 1.

NOTE: Other protocol defined Inclusion/Exclusion criteria may apply

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