MC4R and obesity due to melanocortin 4 receptor deficiency: We used homologous recombination in embryonic stem (ES) cells to replace the mouse mc4r coding gene by either the human WT MC4R tagged at the N-terminus with a c-myc epitope or a mutant form of the human MC4R carrying an obesity-linked mutation at position R165→W in the hMC4R coding sequence tagged at the N-terminus with 3 tandem sequences of the HA epitope.