 Tetra-primer amplification refractory mutation system PCR, or ARMS-PCR, employs two pairs of primers to amplify two alleles in one PCR reaction.The primers are designed such that the two primer pairs overlap at a SNP location but each match perfectly to only one of the possible SNPs.The basis of the invention is that unexpectedly, oligonucleotides with a mismatched 3'-residue will not function as primers in the PCR under appropriate conditions.