 The lungs are not protected in either of these manners.The detoxification enzymes cytochrome P450 1A1 (CYP1A1) and cytochrome P450 1B1 (CYP1B1) are both protective and necessary for benzo["a"]pyrene toxicity.Experiments with strains of mice engineered to remove (knockout) CYP1A1 and CYP1B1 reveal that CYP1A1 primarily acts to protect mammals from low doses of B"a"P, and that removing this protection accumulates large concentrations of B"a"P. Unless CYP1B1 is also knocked out, toxicity results from the bioactivation of B"a"P to benzo["a"]pyrene -7,8-dihydrodiol-9,10-epoxide, the ultimate toxic compound.