McLean, K. L. and Schmutz, S. M. 2011. Melanocortin 4 receptor polymorphism is associated with carcass fat in beef cattle. Can. J. Anim. Sci. 91: 75–79. Melanocortin 4 receptor (MC4R) binds a-melanocyte stimulating hormone (a-MSH) reducing feed and energy intake in several species of animals. One variant in swine has been reported to increase daily gain, backfat deposition and feed intake. MC4R sequence was obtained from 20 random crossbred steers where a novel Ser330Asn polymorphism was detected. Three hundred and eighty-two crossbred Canadian steers and 985 crossbred American steers were genotyped for this polymorphism. The Ser330Asn polymorphism had a minor allele frequency of 0.01 in the Canadian and 0.02 in the American steer populations. The Canadian steers with the heterozygous genotype had increased grade fat (P=0.036) and decreased lean meat yield (P=0.032). The American heterozygous steers had increased backfat (P=0.031) and less desirable yield grades (P=0.022), but also lower longissimus dorsi measurements (P=0.031). The association of the Asn330 allele was validated in two typical crossbred steer populations in two countries, suggesting it has effects of commercial significance.