FGF21 and obesity due to melanocortin 4 receptor deficiency: To address this hypothesis, we conducted a deep phenotypic characterization of the Titan mice and investigated factors involved in detrimental obesity in humans or other model species such as plasma levels of cholesterol and triglycerides16, leptin16–18, fibroblast growth factor 21 (FGF21)19,20, heart morphology21, and alterations of the adipose tissue22.