OPRM1 and obesity due to melanocortin 4 receptor deficiency: Although MOR antagonists are currently used to treat obesity (Apovian, 2016; Ziauddeen et al., 2013), a more refined understanding of the impact of endogenous opioids on appetitive behaviors is required to understand how these drugs work, and to identify future targets for more specific and effective treatments that reduce preference for calorie-dense or high-fat foods.