Our knowledge of the genetic factors affecting obesity is increasing, butinformation about the individual gene effects remains limited in humans as wellas in animal models. The melanocortin-4 receptor gene (MC4R) has been implicatedin the regulation of feeding behavior and body weight in humans and mice. Wehave studied MC4R as a candidate gene for the control of economically importantgrowth and performance traits in the pig. A missense mutation was identified ina region highly conserved among melanocortin receptor (MCR) genes. To determinewhether there was an association of this MC4R polymorphism with phenotypicvariation, we tested the mutation in a large number of individual animals fromseveral different pig lines. Analyses of growth and performance test recordsshowed significant associations of MC4R genotypes with backfat and growth ratein a number of lines as well as feed intake overall. It is probable that thevariant amino acid residue of the MC4R mutation (or a closely linked mutation)causes a significant change of the MC4R function. These results support thefunctional significance of a pig MC4R missense mutation and suggest thatcomparative genomics based on model species may be equally important forapplication to farm animals as they are for human medicine.