PASA; a method for genotyping of single-base mutations. Genomic DNA is used as a template in two reactions, one with a primer that makes a perfect match to the wild-type allele, another with a primer matched to the mutated allele; both reactions contain a second primer that makes a perfect match to the opposite strand of either wild-type or mutant allele. PCR will preferentially amplify fragments from an allele which is present in the genomic DNA. Characteristic results will be seen for wild-type and mutant homozygotes and for heterozygotes.