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
