Growth Differentiation Factor 15 (GDF-15), previously proposed to be a potential biomarker of mitochondrial disease,27,28 was 1,655 ± 262.2 pg/mL (Mayo Clinic Laboratories, normal ≤ 750 pg/mL) in MM (n = 44) and 524.9 ± 50.7 pg/mL in the Unlikely group (n = 20), p = 0.003. Here, GDF15 is linked to mitochondrial disease.