The mechanisms that balance food intake and energy expenditure determine who will be obese and who will be lean. One of the molecules that regulates energy balance in the mouse is the obese (ob) gene. Mutation ofobresults in profound obesity and type II diabetes as part of a syndrome that resembles morbid obesity in humans. The ob gene product may function as part of a signalling pathway from adipose tissue that acts to regulate the size of the body fat depot.