FGF21 and obesity due to melanocortin 4 receptor deficiency: Li et al. [37] demonstrated that RARβ promotes fatty acid oxidation in a mouse model of obesity-associated metabolic disease, but while these authors invoked increases in FGF21 as the major mechanism by which RARβ acted in this murine model, we did not detect lower FGF21 mRNA levels in the BKO livers compared to the WT livers (Figure 2).