There is also evidence that ATF4 regulates obesity, energy expenditure, and glucose homeostasis, in that ATF4-null mice are lean and mildly hypoglycemic and resist obesity induced by aging or high energy diets (Yoshizawa et al., 2009; Wang et al., 2010; Miller et al., 2011). Here, ATF4 is linked to obesity due to melanocortin 4 receptor deficiency.