Current methodology in gene cloning depends largely on the specific cleavage of DNA at so-called restriction endonuclease sites defined by specific base sequences. Since such sequences are not under control of the experimenter, a procedure is proposed by which cleavage at any desired nucleotide sequence in DNA becomes possible. An oligonucleotide complementary to a desired section of either strand of a given DNA molecule is synthesized chemically and annealed to the DNA, resulting in the generation of a single-stranded loop which is cleaved with S1-nuclease (an enzyme which cleaves single-stranded DNA); the oligomer is then removed (i. e., by differential melting) and the newly exposed single-stranded DNA region is again cleaved with the nuclease.