Safety of the proposed transition paradigms is defined as both traditional safety measures but also recurrence of disease activity during a washout period of 8 weeks after the cessation of natalizumab treatment followed by fingolimod treatment initiation. An interim analysis of the first 15 patients revealed a clinical and MRI activity in 38.5 and 64.3% respectively. Therefore a shorter interval was defined for any subject entering into this study after october 2013: the interval was defined to be 4 weeks for those patients.