Background: Approximately 10％ to 30％ of colorectal cancers exhibited mutations in the phosphoinositide-3-kinase,catalytic, alpha polypeptide gene (PIK3CA).However, their detection was limited because of the low sensitivity of conventional sequencing techniques.Furthermore, considering that a majority of colorectal cancer patients were impossible to continuously monitor the progress of tumor by biopsy, peripheral blood could be the only practical sample for detection of mutations.Peptide nucleic acid (PNA) was a synthetic nucleic acid analogue which could combine with the single DNA template, but not served as a primer to guide the duplication of DNA.Compared with the ordinary nucleic, the annealing temperature (Tm) of PNA was 50％ higher.However, when there was a base pair mismatch between PNA and template nucleic the binding force would drop rapidly.Taking advantage of these unique properties of PNA, it could be used to block chain elongation and primer annealing in PCR without interfering with the reactions of mismatched template DNA.Therefore, PNA-PCR induced preferential amplification of mutant DNA fragments even in the presence of an excess amount of wild-type DNA.