LEP and obesity due to melanocortin 4 receptor deficiency: Among various animal models that have been developed and are routinely used to study the pathogenesis and mechanisms of obesity/T2D, high-fat-diet-induced obese (DIO) mice and genetically induced obese mice, i.e., leptin-deficient (Lepob/ob; hereafter, ObOb) mice and leptin-receptor-deficient (Lepdb/db; hereafter, DbDb) mice remain the most widely used experimental models.