A procedure for copying and thereby amplifying the sequence of a defined region of a DNA molecule. The polymerase chain reaction (PCR) generates millions of new DNA molecules all with sequences identical to the region of the original DNA molecule. This procedure was developed by Kary Mullis in 1983 and rapidly became one of the most important tools used by molecular biologists. In addition to its use in scientific research, PCR has many wide-ranging applications, including clinical diagnosis, paternity testing, and forensic science.