What is the purpose of this study? The purpose of this study is to measure nitric oxide in individuals with MELAS and see if giving arginine or citrulline will increase the formation of nitric oxide. Nitric oxide is thought to be helpful in preventing strokes. Therefore, if arginine and/or citrulline are shown to increase the formation of nitric oxide, they could be used to prevent or treat the strokes in patients with MELAS syndrome.