For example, the ob/ob [obese], db/db [diabetes] and the Zucker diabetic fatty rat (ZDF) have mutations either in the leptin gene (ob/ob) or in the leptin receptor (db/db) which causes over-eating obesity, unlike the natural history of the disease in humans where leptin is seldom implicated [37,38]. The gene discussed is LEPR; the disease is obesity due to melanocortin 4 receptor deficiency.