Nucleic acid analogues such as peptide-nucleic acids (PNAs) which hybridize strongly to nucleic acids are used to inhibit nucleic acid amplification procedures such as PCR. False positives in subsequent PCR assays are prevented by hybridizing a PNA to PCR amplification products. Assays capable of discriminating between single base mutants are conducted by using a PNA hybridizing to one of the two allelic forms to inhibit a PCR amplification of that form selectively. Asymmetric PCR amplifications are carried out by starting a PCR symmetrically using like quantities of forward and reverse primers, and, once the amplification is established, disabling one primer by hybridizing a PNA thereto.