FGF21 and Mitochondrial myopathy: Interestingly, in human patients with mitochondrial myopathy caused by OXPHOS deficiencies, the cytokine, fibroblast growth factor 21 (FGF21), is secreted from muscle tissues, suggesting that OXPHOS dysfunction may mimic fasting and induce the secretion of the fasting-induced hormone, FGF21 [16].