PCR is an acronym for polymerase chain reaction, a method for amplifying specific DNA fragments in vitro by the action of a series of enzymes. The main steps are denaturation at high temperature, annealing at low temperature and extension at moderate temperature. The three steps form a cycle that is repeated 35-40 times, resulting in exponential amplification of the target DNA fragment. The assay requires only a small amount of DNA to produce results and is characterized by high specificity, high sensitivity, simple operation and time saving. It can be used for basic research such as gene isolation, cloning, and analysis of nucleic acid sequences. It is also widely used in clinical applications, such as hepatitis B DNA testing, hepatitis C RNA testing and genotyping of hepatitis B and C, as well as in the diagnosis of sexually transmitted diseases and genetic diseases.