Cellular polynucleotides are alkylated by endogenous components, such as S-adenosylmethionine, or after reacting with two general classes of environmental and laboratory chemicals. SN1 chemical agents include alkylnitrosourea and N-alkyl-N-nitro-N-nitrosoguanidine that react with the N7 position of guanine, N3 of adenine, O6 of guanine, O2 or O4 of pyrimidines, and the non-phosphodiester oxygen atoms of the phosphate backbone. In contrast, SN2 chemical agents such as methyl methanesulfonate and dimethyl sulfate react primarily with the N1 position of adenine (1-Methyl-2'-deoxyadenosine) and N3 of cytosine.