MMP9 and breast cancer: Exposure of MCF-7 breast cancer cells to environmental carcinogens: 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK), benzo[a]pyrene (B[a]P) and 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP) increased the expression of the EMT markers MMP-9 and vimentin while diminishing the level of E-cadherin.