Validating a peptide-based laboratory test enabling the measurement of Tysabri® serum levels (pharmacokinetics, PK) in multiple sclerosis patients undergoing therapy. The results of this newly developed test will be compared to Tysabri® serum levels measured in parallel using an independent test. We will also simultaneously measure a pharmacodynamic marker of Tysabri®, receptor saturation levels on blood immune cells, which is thought to correlate with Tysabri® serum levels.

From a single site, consenting patients with relapsing forms of multiple sclerosis who are currently begin treated with Tysabri® (natalizumab) and meeting the eligibility criteria will be asked to provide blood samples on the same day, and prior to, a scheduled infusion of Tysabri® 300 mg IV. Up to 500 patients will be enrolled in this study. Each patient may be asked to donate blood up to 2 times (one blood draw per cycle) to obtain longitudinal data points. Each patient will also be asked to fill out a wellness patient reported outcome (PRO) questionnaire along with the blood draws

Inclusion Criteria:

• Infusing with Tysabri at a single site, Rocky Mountain MS Clinic

Exclusion Criteria:

• If subject donated or received blood, plasma, or any other blood products between the last Tysabri® infusion and the time of sample collection.

Only patients with multiple sclerosis who have had one or more doses of Tysabri® (natalizumab) and are currently being treated with Tysabri® will be included in this study.