MC4R and obesity due to melanocortin 4 receptor deficiency: Key insights into such appetite-regulating genes stem from studies on single gene defects that strongly associate with obesity, including those in key genes such as proopiomelanocortin (POMC) and melanocortin 4 receptor (MC4R) [17] phenotypically characterised by intractable hunger and the development of severe obesity from an early age.