A method is described for the investigation of cytosine methylation in DNA sequences. Triplex-forming oligomers are utilized, which preferably form triplex structures at positions where cytosine unmethylated at position 5 is present. The triplexes block the transcription, replication and amplification of the DNA. In particular, peptide nucleic acid oligomers with modified nucleobases can be used as triplex-forming oligomers.