As a proof of concept, PRL treatment in mice and rats with streptozotocin (STZ)-induced diabetes or diet-induced obesity improves their metabolic profile (2, 11, 12), whereas PRLR null mice with STZ-induced diabetes or diet-induced obesity show a more severe disease phenotype (2, 13). This evidence concerns the gene PRLR and obesity due to melanocortin 4 receptor deficiency.