FGF21 and Mitochondrial myopathy: FGF21 expression is low in healthy skeletal muscle; however, fasting, endoplasmic reticulum stress, mitochondrial myopathy, and metabolic disorders induce increased expression of FGF21 in skeletal muscle, especially in dystrophic mice, where mRNA and protein expression of skeletal muscle FGF21 are significantly upregulated (120–122).