The chemical insertion of 2,2-diamino-4-[(2-deoxy-beta-D-erythro-pentofuranosyl)amino]-5(2H)-oxazolone (oxazolone), a major oxidation compound of 2＇-deoxyguanosine, into oligonucleotides is a challenging issue, due to the high alkali lability of the lesion. This difficulty was overcome by the specific riboflavin-mediated photosensitization of a central guanine residue in DNA fragments. The modified 9-mer thus prepared, was purified by reverse HPLC and characterized by electrospray ionization mass spectrometry. Additional structural information was gained from the assignment of enzymatic hydrolysates of the oxidized DNA fragment. The high alkali lability of the oxazolone-containing DNA fragment was inferred from sequence gel analysis. In contrast, a similar 9-mer that contains 8-oxo-7,8-dihydro-2＇-deoxyguanosine in place of the oxazolone was fully stable under the conditions of piperidine treatment.