LEP and obesity due to melanocortin 4 receptor deficiency: Although currently such a factor remains to be identified, this model is reminiscent of global DGAT1 knockout mice (Smith et al., 2000), which exhibit increased energy expenditure, enhanced glucose metabolism, protection from diet-induced obesity, and increased leptin sensitivity (Chen et al., 2002; Chen et al., 2003; Chen et al., 2004).