Serial stool samples will be collected from each subject, and sent to Second Genome for analysis. The first stool sample will be collected prior to the initial dose of DMF, with subsequent collections at defined time points over the course of the study. A more intensive analysis will focus on the first 12 weeks of treatment, a time during which development and resolution of gastrointestinal side effects typically take place on therapy. Treatment-emergent flushing severity will be recorded during this time using a 5-point Likert scale flushing severity survey. Gastrointestinal (G.I.) symptoms will be assessed using the Gastrointestinal Symptoms Rating Scale (GSRS) questionnaire, a validated, self-administered questionnaire that includes 15 questions, which assess severity of G.I. symptoms using a 7-point Likert scale in five domains: indigestion, diarrhea, constipation, abdominal pain and reflux. The severity of symptoms reported in the GSRS increases with increasing score. Other variables which potentially may alter the gastrointestinal microbiota and secondarily DMF tolerance will be assessed, including the identification of subjects predisposed to functional bowel disorders via use of the Rome III functional bowel survey, a validated clinical tool to identify at-risk individuals, diet composition, antibiotic exposure, steroid treatments for neurological relapses, use of prebiotic , probiotic, or vitamin D supplements, and H2 blocker or proton pump inhibitor (PPI) therapy, as gut pH changes impact the gut flora composition.Additional data on mood change over 24 weeks of DMF therapy, using the Hamilton Anxiety Measurement (HAM) Rating Scale and Patient Health Questionnaire-9 (PHQ-9) Depression Scale, will be collected and correlated to DMF-emergent G.I. disturbances, and to changes in bacterial and archaeal species in the gut flora.