Leptin, the obese (ob) gene product, is a small protein expressed in and secreted from adipose tissue of normal rodents. Studies suggest leptin acts as a circulating hormone capable of regulating body-weight homeostasis and energy balance. One possible target tissue for leptin is the hypothalamus, a proposed control center for satiety and energy expenditure. Ob gene knockout mice are characterized by several metabolic abnormalities including hyperglucocorticoidemia, hyperinsulinemia and insulin resistance, hyperglycemia, altered central nervous system activity, reduced metabolic rate of brown adipose tissue, and a large increase in white adipose tissue. Studies show that the administration of recombinant leptin to ob knockout mice reduces food intake and increases energy expenditure.