GCGR and obesity due to melanocortin 4 receptor deficiency: Gcgr −/− are viable, but have reduced plasma glucose levels, improved glucose tolerance, are resistant to diet-induced obesity, have elevated GLP-1 levels, and do not have normal fat metabolic control during a fast (Parker et al. 2002; Gelling et al. 2003; Sorensen et al. 2006; Vuguin et al. 2006; Conarello et al. 2007; Longuet et al. 2008).