FGF21 and obesity due to melanocortin 4 receptor deficiency: FGF21 is an endocrine hormone primarily synthesized by the liver and adipose tissue, regulated by peroxisome proliferator-activated receptors (PPAR)δ and PPARγ, that plays a role in glucose metabolism, fat, insulin resistance, and obesity (Gómez-Sámano et al., 2017; Fisher & Maratos-Flier, 2016; Ornitz & Itoh, 2015; Tomita et al., 2020b).