MC4R and obesity due to melanocortin 4 receptor deficiency: For example, the MT2R mutations in ligand binding and G protein activation are associated with type 2 diabetes (87), and MC4R deficiency is prevalently found in obesity (88) (please refer to sections GPCRs for Sensing Circulatory Inputs and GPCR-Mediated Autocrine and Paracrine Regulations in the Hypothalamus for Energy Homeostasis, respectively).