Recently, it was clarified that a deficiency of or resistance to leptin, a 16 kDa protein hormone that plays a key role in regulating energy intake and energy expenditure, caused the development of obesity in ob/ob mice (this is a model of ob gene deficient) [25], fa/fa rats, and db/db mice (which are leptin-resistant models due to a mutation of the leptin receptor) [26, 27]. Here, LEP is linked to obesity due to melanocortin 4 receptor deficiency.