Cleary et al. demonstrated that two in vivo models of obesity, the mouse mammary tumor virus (MMTV)-TGF-α/Lep(ob)/(ob) (leptin-deficient) and MMTV-TGF-α/ObR(db)/(db) (leptin receptor-deficient) mice, did not develop mammary tumors compared to wild-type mice [178,179]. This evidence concerns the gene LEP and obesity due to melanocortin 4 receptor deficiency.