A causal link between leptin and breast cancer is supported by animal studies in which obese mice that overexpressed transforming growth factor-alpha (TGF-α), but were deficient in leptin, did not develop mammary tumors, while heterozygous and homozygous wild type leptin mice developed tumors in 50% and 67% of cases, respectively [67]. The gene discussed is LEP; the disease is breast cancer.