Thus, FGF21 has recently gained attention as a potential biomarker of mitochondrial diseases (Tyynismaa et al., 2010; Suomalainen et al., 2011; Lehtonen et al., 2016, 2020) and could represent a potential target for the treatment of mitochondrial myopathies and muscle mitochondria dysfunction. The gene discussed is FGF21; the disease is mitochondrial disease.