 the application of a 32p - postlabeling assay for 7-methylguanines in dna was studied either by labeling the imidazole ring - opened dinucleotide derivatives or by using strong - anion - exchange column chromatography for the adduct enrichment from normal nucleotides . 
 data showed that 7-methylguanines can be efficiently labeled as dinucleotides when in vitro methylated dna was first imidazole ring - opened and then digested to the dinucleotide level with deoxyribonuclease i , snake venom phosphodiesterase , and prostatic acid phosphatase . when using ion exchange chromatography for the adduct enrichment , dna was digested with micrococcal nuclease and spleen phosphodiesterase . 
 anion exchange chromatography was applied for 7-methylguanine measurements in white blood cell dna of healthy nonsmokers ( n = 17 ) and patients ( n = 4 ) treated with the methylating drugs procarbazine and decarbazine . 
 we found that the mean level of 7-methylguanine residues in nonsmokers was 2.5 per 10(7 ) nucleotides . the corresponding level in the patient samples immediately after the drug treatment was 57 per 10(7 ) nucleotides.imagesfigure 2 . 
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