When fed with a HFD, these mice gain similar weight to their wild-type (WT) littermate controls [(Osei-Hyiaman et al., 2008) and Figure 1F]; however, they are less prone to develop liver steatosis, dyslipidemia, and leptin resistance (Osei-Hyiaman et al., 2008), making hepatic CB1R a central regulator of obesity-related liver complications. The gene discussed is LEP; the disease is obesity due to melanocortin 4 receptor deficiency.