INS and obesity due to melanocortin 4 receptor deficiency: Db/db mouse is one of the most characteristic T2DM models induced by obesity, which results from the monogenic mutation in leptin receptor gene and becomes hyperphagic, obese (about at the age of 4 weeks), hyperinsulinemic (about at the age of 2 weeks) and insulin resistant, but later (4–8 weeks) develops hyperglycaemia and does not live longer than 8–10 months [20].