An assay for the cytosine methylation level in the eukaryotic DNA CCWGG sequence is proposed. The method is based on the ability of DNA methylase BstNI to methylate DNA containing in a CCWGG site a nonmodified or 5-methylated cytosine to yield N4-methyl- or N4,5-dimethylcytosine, respectively. © 1999 MAEe cyrillic signK "Hayκa/Interperiodica".