MC4R and obesity due to melanocortin 4 receptor deficiency: In humans, haploinsufficiency, defined as the loss of one copy of a gene due to mutation, of the MC4R is the most common cause of monogenic obesity (Farooqi et al. 2000; Vaisse et al. 2000; Lubrano-Berthelier et al. 2006; Calton et al. 2009).