The polymerase chain reaction (PCR) has revolutionized molecular biology. Portions of single-copy per cell genes (and cDNAs) prepared from very small tissue or cell samples can be specifically amplified for use in sequence determination, gene identification, and quantitation. Improvements to the method, such as the introduction of genetically engineered, thermostable polymerases, more precise thermocyclers and more efficient reverse transcriptases for mRNA conversion to cDNA, have combined to make RNA-PCR (also called reverse transcriptase, or RT-PCR) and PCR more reproducible and specific. Coupled with the high sensitivity of the reactions, RT-PCR and PCR are increasingly used as quantitative bio-analytical techniques.