The polymerase chain reaction (PCR) is a powerful tool for the amplification of trace amounts of nucleic acids, and has rapidly become an essential analytical tool for virtually all aspects of biological research in experimental biology and medicine. Because the application of this technique provides unprecedented sensitivity, it has facilitated the development of a variety of nucleic acid-based systems for diagnostic purposes, such as the detection of viral (1 ) or bacterial pathogens (2 ), as well as genetic disorders (3 ), cancer (4 ), and forensic analysis (5 ). These recently developed systems open up the possibility of performing reliable diagnosis even before any symptoms of the disease appear, thus considerably improving the chances of success with treatment. For many routine applications, particularly in the diagnosis of viral infections, the required answer is the presence or the absence of a given sequence in a given sample. Therefore, PCR is in able for the early diagnosis of HCV infection (6 ), HSV encephalitis (7 ), or HIV infection of babies of HIV-positive mothers (8 ). On the other hand, since even minute amounts of DNA are detected, the medical interpretation of positive results for widespread infectious agents like CMV (9 ) or HHV6 (10 ) turned out to be rather difficult.
