Obesity in Ay mice is thought to be a consequence of the agouti protein acting as a constitutive antagonist of the melanocortin 3 receptors (MC3R) and melanocortin 4 receptor (MC4R) by mimicking the actions of the agouti-related protein[9-11]. The gene discussed is MC3R; the disease is obesity due to melanocortin 4 receptor deficiency.