The main goal of this study is to learn how [14C]-BIIB091 moves through and is processed by the body and to look at how much of BIIB091's metabolites (what is produced when BIIB091 is broken down by the body) appears in the blood, urine, and stool in healthy male participants. The study will also help researchers learn more about the safety of BIIB091 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.

In accordance with Biogen's Clinical Trial Transparency and Data Sharing Policy on https://www.biogentrialtransparency.com/