Nonradioactive oligonucleotide probes are increasingly being used in various applications, for example, polymerase chain reaction (PCR) product detection, in situ hybridization and colony and plaque screening (1 ). Oligonucleotide probes are short, defined sequences, 15–50 bases long, that are relatively easy to produce. As the base composition of oligonucleotide probes dictates the hybridization properties (2 ), probes can be designed to discriminate between perfectly matched and mismatched target sequences.