FGF21 and Atrophy: Although marginal, inhibition of the elevated FGF21 expression from skeletal muscle in Opa-1−/− mice, via skeletal muscle-specific FGF21 KO, demonstrated beneficial effects on muscle mass (Tezze et al., 2017), which suggests that FGF21 might be a causative factor or mediator in the muscle atrophy observed in this mitochondrial deficiency animal model.