AGRP and obesity due to melanocortin 4 receptor deficiency: Mechanisms mediating hyperphagia and obesity in this model involve antagonism of the melanocortin receptor, and similar phenotypes have been described in mice overexpressing Agouti-Related protein (AGRP), a potent antagonist of the melanocortin receptors-3 and -4 (Moussa and Claycombe, 1999; Small et al., 2001).