Some of the common mitochondrial disorders encountered in adult neurologic practice are mitochondrial encephalomyopathy with lactic acidosis and stroke-like episodes (MELAS), Kearns–Sayre syndrome, Leigh disease, mitochondrial neurogastrointestinal encephalomyopathy (MNGIE), myoclonic epilepsy with ragged red fibers (MERRF), Leber hereditary optic neuropathy, pyruvate dehydrogenase complex deficiency, polymerase gamma (POLG)-related disorders, and various other mitochondrial DNA depletion syndromes (95). The gene discussed is POLG; the disease is MERRF.