It has been reported that obese rodents and individuals have lower circulating levels of this cytokine [23]–[25] and treatment with IL-15 induces weight loss in wildtype C57BL/6 (B6) mice [26], IL-15−/− mice, and murine models of obesity [25], [27]. The gene discussed is IL15; the disease is obesity due to melanocortin 4 receptor deficiency.