Melanocortin 4 receptor (MC4R) haploinsufficiency is the most common monogenic cause of obesity, with a prevalence of 1.7–3.0 % in obese individuals (Calton et al. 2009; Hainerová et al. 2007; Hinney et al. 2006). The gene discussed is MC4R; the disease is obesity due to melanocortin 4 receptor deficiency.