First described by Newton and colleagues in 1989 (1 ), amplification refractory mutations system (ARMS) has become a standard technique that allows the discrimination of alleles that differ by as little as 1 bp. The system is simple, reliable, and nonisotopic. It clearly distinguishes heterozygotes at a locus from homozygotes for either allele. The system requires neither restriction enzyme digestion, nor allele-specific oligonucleotides as conventionally applied, nor the sequence analysis of polymerase chain reaction (PCR) products. The basis of the system is that oligonucleotides with a mismatched 3′-residue will not function as primers in the PCR under appropriate conditions.