A method for cleavage of double-stranded DNA at specific restriction sites. A suitable restriction site is selected, around which the base sequence is known, and an oligonucleotide, up to 50 or 60 bases in length, is synthesised to complement it. It is hybridised to the DNA to form a triple helix, which is stabilised with the help of the RecA protein. Methylation then blocks all but the protected restriction site, so that subsequent treatment with the restriction nuclease, in the absence of the protecting oligonucleotide and RecA protein, will cleave only at the designated site. The use of two protecting oligonucleotides for sites that span a sequence of interest allows excision of a specific fragment from a DNA preparation as complex as the human genome.