The purpose of this study is to investigate the neurological and biochemical consequences of the m.3243 A>G mutation. Mitochondria are the powerhouses of the cell and are controlled by nuclear genetic material (DNA) and mitochondrial (mt) DNA. Mitochondrial DNA mutations impair mitochondrial function, and cause cellular energy failure. These mutations, when present in high abundance, cause neurological signs and symptoms that are clinically obvious. The investigators hypothesize that these mutations, when present in lesser abundance, will cause measurable alterations in the patient's neuropsychological profile and cerebral energy profile. This study does not involve any experimental or approved therapy. The investigators will evaluate the patient's condition with blood/urine tests, neurological exam, MRI/MRS, questionnaires, motor skills functioning, serum and urine biomarkers, and genetic testing.