Pharmacokinetic, Immunological and Biochemical Sample Collection and Analysis of a Tysabri Patient Cohort

Inclusion Criteria: 1. Ability to understand the purpose and risks of the study and provide signed and dated informed consent and authorization to use protected health information (PHI) in accordance with national and local subject privacy regulations. 2. Aged 18 to 75 years old, inclusive, at the time of informed consent. 3. Must be a patient with a relapsing form of Multiple Sclerosis enrolled in the TOUCH Prescribing Program who is not expected to discontinue Tysabri® therapy prior to completion of the requirements of this study. 4. Must have been treated with monthly IV infusions of Tysabri® 300 mg for at least 1 month. 5. Must have a magnetic resonance imaging (MRI) brain scan, performed prior to the initiation of treatment with Tysabri®, on file. 6. Must weigh between 42 and 126 kg, inclusive. 7. Fifteen patients who are 1 month to-12 months of natalizumab therapy, fifteen patients who are 24 months and over of natalizumab therapy will be asked to participate in the TNF alpha subset. Exclusion Criteria: Medical History: 1. History of, or abnormal laboratory results indicative of, any clinically significant cardiac, endocrinologic, hematologic, hepatic, immunologic, metabolic, urologic, pulmonary, gastrointestinal, dermatologic, psychiatric, and renal, or other major disease, as determined by the Investigator. 2. If subject answers 'Yes" to any question on the PML questionnaire that is not resolved prior to infusion as per standard operating procedure for natalizumab infusion.