MC4R and obesity due to melanocortin 4 receptor deficiency: Deletion of the Mc4r in mice fed a standard lab chow diet exhibit a mild obesity phenotype (Butler et al., 2001), which manifests as an aberrant state of energy balance characterized by disruptions in ingestive behavior (hyperphagia), a dysregulated metabolic phenotype (hyperglycemia, hyperinsulinemia, dyslipidemia), as well as massive hepatic steatosis when fed a high fat diet (Huszar et al., 1997; Marsh et al., 1999a; Rahmouni et al., 2003; Barb et al., 2004; Balthasar et al., 2005; Cone, 2005; Butler, 2006; Itoh et al., 2011).