A method and an oligonucleotide probe are described for determining the presence or absence of mutant alleles in a genomic locus. The probe binds to different alleles of a target sequence with different melting temperatures (Tm). The method determines the Tm of the probe when it is hybridized to the target sequence to establish whether a variant nucleic acid such as a mutant allele is present or absent in the target sequence. There may be variants in a target sequence that are not of interest, for example phenotypically silent mutations. To ensure that these variants do not influence the Tm of the probe, the probe contains universal base sites where such variants of no interest occur.