No treatment is provided in this observational, longitudinal cohort study. Study population will consist of United States (US) participants with relapsing MS receiving commercial natalizumab. There are no study-mandated visits. Serum samples will be collected during routine patient care or follow-up visits and will be sent to a central laboratory for analysis (presence of anti-JCV antibody), and remaining serum aliquots will be stored for future natalizumab and PML research. Additional samples will be collected at participating sites from participants who qualify and consent to participate in focused sampling group (participants who are anti-JCV antibody positive at any time point AND have received ≥12 infusions of natalizumab, whether or not they have a history of immunosuppressant use). These samples will be stored for future natalizumab and PML research.
