PCR (polymerase chain reaction) is a technique used to amplify a single copy of a DNA segment, generating thousands to millions of copies. Developed by Kary Mullis in 1983, PCR uses thermal cycling to denature DNA, allow primers to anneal, and extend new strands using a thermostable polymerase. PCR is now commonly used in clinical and research applications such as disease diagnosis, genetic testing, forensics, and studies of evolution.