It bears emphasis that the current studies were undertaken in two genotypes of mice fed low fat rodent chow, suggesting that the increased fatty acid flux in Mttp-IKO mice resulted from the unabsorbed lipid rather than from increased dietary fat intake or from obesity (since Mttp-IKO mice are lean), or from increased colonic accumulation of neutral lipid. The gene discussed is MT-TP; the disease is obesity due to melanocortin 4 receptor deficiency.