All oxygens in nucleic acids react with carcinogenic ethylating agents.

Over 80% of ethylnitrosourea and ethylnitrosoguanidine modification of nucleic acids is on oxygens. The reactivity of oxygens (other than ribose and phosphate) in single-stranded RNA is: O2 of C greater than O2 of U greater than O6 of G greater than O4 of U. In double-stranded DNA the order is: O2 of T equals O6 of G greater than O4 of T greater than O2 of C. Oxygen reactivity of single-stranded DNA resembles RNA. The glycosidic bond of O2-alkylpyrimidines is labilised.