Previous studies from our laboratory reported that leptin and leptin receptor-deficient mice do not develop mammary tumors (MTs): when Lep strain mice were crossbred with transgenic MMTV-TGF-a mice, obese MMTV-TGF-α/Lep(ob)Lep(ob) mice did not develop oncogene-induced MTs, although their lean littermates did [10]. The gene discussed is LEP; the disease is breast cancer.