The invention relates to an in vitro method for selectively amplifying a target DNA sequence from a nucleic acid sample, which method comprises running a PCR amplification of a nucleic acid sample suspected of comprising at least one target DNA sequence that differs from a reference DNA sequence at at least one predetermined target mutation site; wherein said method employs a blocking oligonucleotide complementary to a portion of the reference DNA sequence comprising the target mutation site, with the exception of a least one mismatch outside the target mutation site.