Leptin (Greek λεπτός meaning thin) is a 16-kDa protein hormone that plays a key role in regulating energy intake and energy expenditure, including appetite/hunger and metabolism. It is one of the most important adipose-derived hormones. The Ob(Lep) gene (Ob for obese, Lep for leptin) is located on chromosome 7 in humans. It functions by binding to the leptin receptor.