FGF21 and obesity due to melanocortin 4 receptor deficiency: In addition to artificially reversing the reduced weight loss in obese mice, fibroblast growth factor 21 (FGF21) gene expression in the liver and hepatocytes can be induced by inhibiting HDACs, thus stimulating the β-oxidation of long-chain fatty acids and the production of ketone bodies, which may be a treatment for obesity (13, 99, 101).