LEP and obesity due to melanocortin 4 receptor deficiency: Leptin, a 16 kDa hormonal product of the obesity (ob) gene, is primarily secreted by adipocytes and regulates food intake and energy homeostasis mainly through its action on the hypothalamus and mice genetically deficient in leptin (ob/ob) or its receptor (db/db) become grossly overweight, hyperphagic and develop severe insulin resistance [151].