LEP and obesity due to melanocortin 4 receptor deficiency: Ablation of Stat3 in the CNS (or in Lepr cells) or the mutation of LepRb Tyr1138 in mice promotes hyperphagic obesity and other defects in leptin action, though these effects do not fully recapitulate the phenotype of LepRb deficiency (Bates et al. 2003; Gao et al. 2004; Jiang et al. 2008; Piper et al. 2008).