A polynucleotide comprising at least the final six nucleotides of one of the following primer sequences, or a sequence complementary thereto: SEQ. ID NOS. 3 to 16, 18, 20 to 33, 35 or 37 to 39. A method of detecting the presence or absence of a mutation in the PIK3CA gene, wherein the mutation is one of H1047R, H1047L, E542K and E545K, and preferably ARMS primers are combined with Scorpion primers.