Since FGF21 is induced in and secreted from skeletal muscle in mitochondrial myopathies and various mitochondrial stressors, it might also act as an adaptive mediator of the muscle mitochondrial stress via activation of pathways that control mitochondrial function (Oost et al., 2019, 2020; Klaus and Ost, 2020). The gene discussed is FGF21; the disease is Mitochondrial myopathy.