At the same time, FGF21 has a higher level in patients with mitochondrial diseases affecting skeletal muscle, which can be used as a biomarker for mitochondrial respiratory chain defects in muscles (Suomalainen et al., 2011; Suomalainen, 2013; Nohara et al., 2019). The gene discussed is FGF21; the disease is inborn mitochondrial metabolism disorder.