Abstract: The present invention relates to an in vitro method for detecting a target DNA sequence in cells using an oligonucleotide which is labelled with a beta-D-galactopyranoside which, on hydrolysis of the glycosidic linkage, forms a water-insoluble dye. The invention relates in particular to an in vitro method for detecting a target DNA sequence from a group of DNA sequences whose members differ from one another in exactly one predetermined nucleotide position, by using an oligonucleotide which is labelled with a beta-D-galactopyranoside which, on hydrolysis of the glycosidic linkage, forms a water-insoluble dye. The invention additionally relates to the use of such an oligonucleotide in DNA hybridization methods. The invention finally relates to kits for carrying out the above methods.