ob/ob mice with mutations in the obese gene are extremely obese and hyperphagic and have characteristic neuroendocrine impairments, including impaired reproduction and low sex hormones, low thyroid hormone, low growth hormone, and elevated glucocorticoids, a pattern also observed in fasting mice. The product of the obese gene, called leptin, was originally characterized on the basis of positional cloning. Injections of leptin not only reversed many of the phenotypes of ob/ob mice, many of the neuroendocrine effects of fasting are also reversed by leptin injection.