When it is absent, hypercholesterolemia, atherosclerosis and obesity have been reported to occur, thus Apolipoprotein E-deficient (ApoE−/−) mice display spontaneously increased inflammation and high levels of cholesterol compared to WT mice, and have traditionally served as a metabolic syndrome model used in cardiovascular research [76]. Here, APOE is linked to obesity due to melanocortin 4 receptor deficiency.