It has been shown that HEK293 cells are transiently transfect with the mutant melanocortin 4 receptor (MC4R) [117,118], a rhodopsin-like GPCR expressed in the hypothalamic pro-opiomelanocortin (POMC) neurons and the gene most commonly linked to obesity [119], which is located on chromosome 18q21.31at an early age [120]. Here, MC4R is linked to obesity due to melanocortin 4 receptor deficiency.