MRAP2 and obesity due to melanocortin 4 receptor deficiency: In the homozygous chr2_a region, the loss of function of the mrap2 (melanocortin-2 receptor accessory protein 2A) gene is associated with severe obesity in many species (humans, zebrafish, rodent: [85]; sea lamprey: [86]; snakehead: [87]), and was shown to be under positive selection in the Chilean farmed rainbow trout population [38].