The screening and starting procedures criteria for Gilenya on a clinical basis and for our study are defined by the Gilenya PI. All patients in our clinic have standardized assessments before starting any disease-modifying therapy. Once the patient decided to start Gielnya on a clinical basis and qualified on a screening, as defined by Gilenya PI, our team will be informed and contact the patient regarding participation in the present study before receiving first dose of Gilenya. Once the informed consent is signed, the patient will follow study procedures, as outlined in the protocol and study evaluation schedule. All subjects will be assessed at 0, 6, 12 and 24 months with the same clinical, 3T MRI and laboratory, and humoral response to herpes viruses examinations. The cognitive assessments and evaluation of environmental risk factors will be performed at 0, 12 and 24 months. Safety will be also assessed at 0, 6, 12 and 24 months of the study.