MC4R and obesity due to melanocortin 4 receptor deficiency: The observation that deletion of the MC4R results in a phenotype that is a virtual carbon copy of POMC-deficient mice (including late onset of obesity and diabetes) strongly supports this idea (Huszar et al., 1997; Yaswen et al., 1999; Butler et al., 2001; Challis et al., 2004).