For example, leptin-deficient (ob/ob) and leptin receptor deficient (db/db) mice have been used for studies of obesity and type II diabetes, demonstrating insulin resistance, hyperleptinemia, hyperphagia and extremely high plasma glucose levels at 6–10 weeks old (Zhang et al., 1994). The gene discussed is LEP; the disease is obesity due to melanocortin 4 receptor deficiency.