Leptin is a 16 kDa protein hormone encoded by the obese (ob) gene with important effects in metabolism and regulation of body weight. Leptin has dual actions, decreasing appetite and increasing energy consumption. The primary effect of leptin appears to be mediated by leptin receptors expressed mainly in the hypothalamus. Mutations in the ob gene or leptin receptor gene causes hyperphagia, reduced energy expenditure, and severe obesity. The assays listed below are based on a double-antibody sandwich technique for sensitive measurement of leptin or leptin receptor.