Cell free circulating-mtDNA (ccf-mtDNA), a new biomarker for mitochondrial diseases [99], and GDF15 levels were significantly increased in patients with mitochondrial encephalomyopathy lactic acidosis stroke-like episodes syndrome (MELAS) and myoclonic epilepsy ragged red fibers (MERRF) syndrome. The gene discussed is GDF15; the disease is MERRF syndrome.