Key Inclusion Criteria:  Documented diagnosis of Relapsing Remitting Multiple Sclerosis (McDonald 2010 Criteria). EDSS ≤ 3.0. Must fulfill Tysabri indication (relapse and MRI criteria). Decision to start treatment with Natalizumab must precede enrollment. Up to four natalizumab infusions.  Key Exclusion Criteria:  Any prior treatment with Natalizumab. Prior imunossupressive treatment (Mitoxantrone, Azathioprine, Methotrexate, Cyclophosphamide, Mycophenolate, Cladribine, Rituximab). Contraindications to treatment with Natalizumab. History of Progressive Multifocal Leukoencephalopathy (PML) or other opportunistic infections, or an increased risk for such infections. Immunocompromised at the time of enrollment. Known active malignancies. Inability to comply with study requirements.  Note: Other protocol defined Inclusion/Exclusion criteria may apply.