MRAP2 (melanocortin 2 receptor accessory protein 2) is a central regulator of energy homeostasis that modulates multiple melanocortin receptors. Primarily functioning in the central nervous system 1, MRAP2 enhances ligand sensitivity and cAMP signaling of MC4R and MC3R, critical receptors controlling appetite and food intake 23. Mechanistically, MRAP2 promotes MC4R localization to neuronal primary cilia, which is essential for long-term energy homeostasis regulation 2. MRAP2 also interacts directly with MC3R in hypothalamic neurons, enhancing cAMP signaling while reducing β-arrestin recruitment and receptor internalization 3. Additionally, MRAP2 may negatively regulate MC2R by competing with MRAP for binding and impairing ACTH receptor signaling 4. Disease relevance is substantial: MRAP2 knockout mice develop severe obesity 1, and rare loss-of-function mutations in humans are associated with hyperphagic obesity, hyperglycemia, and hypertension 4. Meta-analysis of seven independent cohorts demonstrated that carriers of rare coding MRAP2 variants had 2.61-fold higher odds of obesity (p=8.0×10⁻⁴) 5. Human genetic variants identified in obese individuals fail to enhance melanocortin receptor signaling 3. MRAP2 thus represents a biologically validated obesity susceptibility gene with therapeutic potential for monogenic hyperphagic obesity 6.