In the current study, we utilized rapid (F344.WKY-Nat2rapid) and slow (F344.WKY-Nat2slow) acetylator rat strains to investigate mammary cancer risk following the administration of methylnitrosourea (MNU) or 7,12-dimethylbenzanthracene (DMBA), neither of which is biotransformed by rat NAT2 [34–37]. This evidence concerns the gene NAT2 and breast cancer.