To assay gene expression or virus genomes in tissues or body fluids, competitive polymerase chain reaction (cPCR) is now performed in many laboratories. cPCR is a quantitative adaption of the PCR method in which a known number of copies of a synthetic RNA (1 ) or DNA (2 –4 ) is coamplified with the target sample and therefore compete for the common primers and reagents in the same reaction tube. After coamplification, both products are distinguished by characteristic features, e.g., size, mostly by electrophoretic methods, or by probe-specific hybridization, e.g., PCR-ELISA (4 ,5 ).