The "polymerase chain reaction" (PCR) is a high-power molecular biology technique allowing in vitro enzymatic amplification of a given DNA sequence. This exponential amplification can reach 10(7)-10(9), even a single DNA molecule can be detected. Also the use of non-radioactive probes, considered to be less sensitive than their radioactive counterparts, is possible for the molecular hybridization, to retain a high level of sensitivity. PCR is defined as a "free bacteria" cloning technique, which has many applications in fundamental research and in the clinical analysis of genetic disease, infectious diseases and cancers. Thus PCR is a revolutionary method which is capable of greatly stimulating scientific research and modifying the diagnostic area in the near future.