A Study to Look at How a Single Oral Dose of Carbon-14-Labelled [14C] BIIB091 Moves Through and is Processed by the Body in Healthy Male Participants

Key Inclusion Criteria:

• Have a body mass index (BMI) of 18 to 32 kilograms per meter square (kg/m^2) and a total body weight greater than (>) 50 kg, as measured at Screening.
• History of regular bowel movements (averaging 1 or more bowel movements per day).
• Negative polymerase chain reaction (PCR) test result for severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) at Day -1.

Key Exclusion Criteria:

• History of any clinically significant cardiac, endocrine, gastrointestinal, hematologic, hepatic, immunologic, metabolic, urologic, pulmonary, neurologic, dermatologic, psychiatric, or renal disease or other major disease, as determined by the Investigator.
• Participants enrolled in a previous radionucleotide study within 12 months prior to screening or who have received radiotherapy within 12 months prior to screening or such that total radioactivity would exceed acceptable dosimetry.
• Chronic, recurrent, or serious infection (e.g., pneumonia, septicemia), as determined by the Investigator, within 90 days prior to Screening or between Screening and Day -1.
• Current enrollment in any other drug, biological, device, or clinical study or treatment with an investigational drug or approved therapy for investigational use within 30 days prior to Day -1 (24 weeks for biologics), or 5 half-lives, whichever is longer.
• Prior exposure to BIIB091 or any lymphocyte-depleting therapy or exposure to any lymphocyte-targeting therapy within 3 months prior to Day -1.

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