Alkylation of DNA. Only the bases of the nucleotide residues are shown. (a) Atoms that are most susceptible to alkylation (circled in red). (b) Benzo[a]pyrene is hydroxylated in the liver to a reactive form (an epoxide) that alkylates nucleotide bases; the product of its reaction with guanine is shown. (c) A nitrogen mustard forms interstrand cross-