INS and obesity due to melanocortin 4 receptor deficiency: Type 2 diabetes mellitus (T2DM) is a complex metabolic disease, which is characterized by insulin resistance of different insulin target tissues (including liver, adipose tissue, and skeletal muscle) and insufficient insulin secretion by pancreatic β-cells, as well as associated with other metabolic diseases, including obesity, dyslipidemia, hyperuricemia and non-alcoholic fatty liver disease [1].