Previously, the fibroblast growth factor 21 (FGF21) and the growth differentiation factor 15 (GDF15) have been used as biomarkers for mitochondrial translation and mtDNA maintenance disorders, or mitochondrial encephalomyopathy, lactic acidosis and stroke-like episodes (MELAS), respectively (48, 49). The gene discussed is FGF21; the disease is MELAS.