Study of Tysabri in Early Relapsing Remitting Multiple Sclerosis Participants

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.