Abstract: A method of diagnosing a KRAS gene mutation at codons 12-13 in a DNA sample is disclosed. The method comprises detecting one or more than one mutation in the KRAS gene codons 12-13 of the DNA sample by performing an allelic discrimination assay using a mutant probe, a wild-type probe paired with the mutant probe, a forward primer and a reverse primer, the mutant probe being adapted to detect a single nucleotide mutation at 1A, 1T, 1C, 2A, 2T, 2C or 5A of the KRAS gene codons 12-13 of the DNA sample, and the primers each having no greater than 25 nucleotides in length are adapted to amplify a region spanning KRAS exon 2 codons 12-13, wherein the mutant and wild-type probes are labeled with different fluorescent dyes.