The primary objective of this study is to demonstrate that the incidence of progressive multifocal Leukoencephalopathy (PML) in natalizumab-treated participants who do not have detectable antibodies to John Cunningham virus (JCV) (antibody negative) is lower than in participants who have detectable antibodies to JCV (antibody positive).

The secondary objectives of this study are to: Estimate the incidence of PML in natalizumab-treated participants who are anti-JCV antibody negative and anti-JCV antibody positive, based on a meta-analysis of data obtained from this study and other data sources; Define the prevalence of anti-JCV antibody in relapsing multiple sclerosis (MS) participants receiving natalizumab within the TYSABRI Outreach: United Commitment to Health (TOUCH) Prescribing Program; Determine changes in anti-JCV antibody status over time.

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.

Key Inclusion Criteria:

• Relapsing MS patients receiving commercial natalizumab
• Patients receiving natalizumab and their prescribers must be enrolled in the TOUCH Prescribing Program.
• Patients with suspected or confirmed PML who are at or referred to a participating STRATIFY-2 site may enroll into STRATIFY-2 for purposes of PML sample collection.

Key Exclusion Criteria:

- Patients may participate in any other clinical trial or study sponsored by Biogen Idec ; however, if the anti-JCV antibody test is included in the other clinical study and that study is performing a longitudinal analysis of those samples, the patient should withdraw from STRATIFY-2.

NOTE: Other protocol defined Inclusion/Exclusion criteria may apply

US participants with relapsing MS receiving commercial natalizumab.