What does the study involve? Individuals with MELAS syndrome will be admitted twice to the General Clinical Research Center (GCRC) at Texas Children's Hospital. Each time they will stay for five days, during which a special diet will be provided. Nitric oxide production will be measured by stable isotopes infusion technique that involves placing small tubes in veins (IV catheter), blood sampling, and injecting a harmless stable isotope. Stable isotopes are forms of normal compounds that can be measured accurately. They are not radioactive and there are no known risks to giving them; they are already part of your body in small amounts.