Recent pharmacological advances and clinical trials show that unimolecular co-agonists that target the receptors for the incretins – GIP and glucagon-like peptide 1 (GLP-1) – offer more effective treatment strategies for obesity and type 2 diabetes mellitus (T2D) compared with GLP-1 receptor (GLP1R) agonists alone, suggesting previously underappreciated roles of GIP in regulating food intake and body weight. This evidence concerns the gene GCG and obesity due to melanocortin 4 receptor deficiency.