All participants enrolled in the study will consume a methionine free diet for 1 week, alternating with temozolomide chemotherapy for 1 week. The participants will then be switched to a methionine "replete" diet for 1 week while taking chemotherapy. This diet and chemotherapy schedule will be continued for as long as the treatment is effective and tolerable. This replete diet contains methionine. The dietician will record what you eat to make sure you are eating the right amount of methionine during this period. This 4-week schedule will be repeated as long as you are on the study.