CPE and obesity due to melanocortin 4 receptor deficiency: This effect of obesity was observed in ob/ob and db/db mice which are obese because of a genetically deficiency in leptin or the leptin receptor, in mice obese because of a genetic deficiency in carboxypeptidase E (Cpe), an enzyme involved in processing neuropeptides related to eating behavior, and in mice diet-induced obesity (5, 50–54).