Abstract: The invention concerns a method for detecting a mutation in a target nucleic acid comprising amplification of the target DNA, specific hybridization of a probe with the DNA target, extension of the probe by selective addition of &agr;S-phosphothioat-edesoxynucleotide complementary of the mutation in a tube A and adding an &agr;S-phosphothioatedesoxynucleotide complementary of the natural base corresponding to said mutation in a tube B; the resulting extended probe being resistant to digestion by an exonuclease, particularly by exonuclease III.

Abstract: The invention concerns a DNA chip system for detecting mutation in a target nucleic acid such that only the DNA comprising the mutation remains on the chip at the end of the process. The invention concerns a method which consists in adding a complementary &agr;S-phosphothioatedesoxynucleotide of the mutation is added by means of DNA polymorase at the 3′ end of the probe hybridised with the target nucleic acid and in adding an exonuclease so that only the elongated probes are not degraded. The detection of the presence or absence of mutation is carried out by directly or indirectly measuring the presence or the absence of DNA in a specific site on the chip. Advantageously, the chip comprises ISFET transistors or piezoelectric transducers.