For example, mice with genetic deficiency of leptin (ob/ob) or leptin receptor gene (db/db) suffers from hyperphagia, severe obesity and diabetes, while administration of recombinant leptin from bacteria reverses these disorders in ob/ob, but not in db/db mice, indicating leptin depends on its receptor for displaying functions [7]. Here, LEP is linked to obesity due to melanocortin 4 receptor deficiency.