Because many metabolic pathways, including insulin signaling17, 18, 19, lipid metabolism20, 21, and food intake22, 23, are conserved between Drosophila and mammals, we used the Drosophila model to investigate the role of TGF-β/Gbb signaling in conditions of nutrient excess or obesity. The gene discussed is INS; the disease is obesity due to melanocortin 4 receptor deficiency.