MRAP2 and obesity due to melanocortin 4 receptor deficiency: Specifically, we found that inactivating Mrap2 specifically in MC4R-expressing neurons causes, like Mc4R loss-of-function, early-onset hyperphagia and obesity, whereas germline inactivation of Mrap2 caused a milder phenotype, with late-onset hyperphagia and obesity (23, 29).