A method of detecting the absence of a wild type allele of a locus in the JAK2 gene having at least first mutant and wild type alleles, the method comprising: a) providing a reaction mix comprising i) a sample including nucleic acid representing at least a portion of the JAK2 gene; ii) an oligonucleotide probe which hybridises to the mutant allele with a lower melting temperature (Tm) than that with which it hybridises to the wild type allele; iii) a pair of oligonucleotide primers for nucleic acid amplification, the primers hybridising to the nucleic acid in the sample at first and second sites flanking the oligonucleotide probe binding site; wherein the Tm of the primer: sample is higher than the Tm of the probe: mutant allele; b) maintaining the reaction mix at a temperature between the probe: mutant allele Tm and the probe: wild type allele Tm, such that the probe preferentially hybridises to the wild type allele; c) carrying out a thermal cycling amplification on the reaction mix, the amplification including a melt phase, an annealing phase, and an extension phase, in which the temperatures of the extension and annealing phases are between the probe: mutant allele Tm and the probe: wild type allele Tm, such that the probe is hybridised to the wild type allele during these phases; to thereby amplify the mutant allele; and d) detecting hybridisation of the probe to the sample at a temperature at or below the probe: mutant allele Tm; detecting hybridisation of the probe to the sample at a higher temperature at or below the probe: wild type allele Tm; and comparing the two; to thereby detect the presence or absence of the wild type allele; and wherein one of the primers is provided in a rate-limiting amount, and the amplification reaction is asymmetric PCR wherein one or both of the primers overlap with the probe binding site such that the probe competes with the primer for binding, to thereby prevent binding of the primer and hence strand extension and wherein the first and second primer binding sites do not overlap the allele locus. a) providing a reaction mix comprising i) a sample including nucleic acid representing at least a portion of the JAK2 gene; ii) an oligonucleotide probe which hybridises to the mutant allele with a lower melting temperature (Tm) than that with which it hybridises to the wild type allele; iii) a pair of oligonucleotide primers for nucleic acid amplification, the primers hybridising to the nucleic acid in the sample at first and second sites flanking the oligonucleotide probe binding site; wherein the Tm of the primer: sample is higher than the Tm of the probe: mutant allele; i) a sample including nucleic acid representing at least a portion of the JAK2 gene; ii) an oligonucleotide probe which hybridises to the mutant allele with a lower melting temperature (Tm) than that with which it hybridises to the wild type allele; iii) a pair of oligonucleotide primers for nucleic acid amplification, the primers hybridising to the nucleic acid in the sample at first and second sites flanking the oligonucleotide probe binding site; wherein the Tm of the primer: sample is higher than the Tm of the probe: mutant allele; b) maintaining the reaction mix at a temperature between the probe: mutant allele Tm and the probe: wild type allele Tm, such that the probe preferentially hybridises to the wild type allele; c) carrying out a thermal cycling amplification on the reaction mix, the amplification including a melt phase, an annealing phase, and an extension phase, in which the temperatures of the extension and annealing phases are between the probe: mutant allele Tm and the probe: wild type allele Tm, such that the probe is hybridised to the wild type allele during these phases; to thereby amplify the mutant allele; and d) detecting hybridisation of the probe to the sample at a temperature at or below the probe: mutant allele Tm; detecting hybridisation of the probe to the sample at a higher temperature at or below the probe: wild type allele Tm; and comparing the two; to thereby detect the presence or absence of the wild type allele; and