MC4R and obesity due to melanocortin 4 receptor deficiency: We and others have reported that mutations in MC4R are found in 5–6 % of patients with severe early-onset obesity (Farooqi et al. 2003; Stutzmann et al. 2008) and at a frequency of approximately 1/1,000 in the general UK population, making this one of the most common human genetic diseases.