Db/db mice have a homozygous spontaneous mutation of the leptin receptor and are a diabetic model that shows type 2 diabetic characteristics such as obesity at 3–4 weeks after birth, an uncontrolled rise in blood glucose and insulin 4–8 weeks after birth, typical diabetic nephropathy and serious loss of pancreatic β cells. The gene discussed is INS; the disease is obesity due to melanocortin 4 receptor deficiency.