MC4R and obesity due to melanocortin 4 receptor deficiency: The most common origin of monogenetic obesity in humans is deficiency, more specifically haploinsufficiency (where a single copy of a gene is inactivated by a mutation), of the MC4R (Farooqi et al. 2000; Vaisse et al. 2000; Lubrano-Berthelier et al. 2006; Calton et al. 2009).