A multiplex method for scanning genomic DNA samples for the presence of specific mutations. PCR primers complementary to the mutant and wild-type sequences are immobilised at coded loci on a surface and exposed to genomic DNA samples. Incorporation of a fluorescent marker during PCR amplification will indicate by the presence of fluorescence at a locus, the existence there of the mutant or wild-type template in the genomic DNA to which it had been exposed.