Serum biomarkers of growth differentiation factor 15 (GDF-15) and fibroblast growth factor 21 (FGF-21) were proposed to aid in decision making as they increase in metabolic diseases with oxidative stress and inflammation; however, they cannot be used as a diagnostic tool for mitochondrial disorders as they increase in a variety of other non-mitochondrial diseases [105]. The gene discussed is FGF21; the disease is mitochondrial disease.