PCR (polymerase chain reaction) is an in vitro technique used to amplify a specific region of DNA across several orders of magnitude, generating thousands to millions of copies of a particular DNA sequence. It involves repeated cycles of heating and cooling of the DNA sample to separate the DNA strands and allow primers to anneal, followed by extension of the primers by a thermostable DNA polymerase. Kary Mullis developed PCR in 1985 and was awarded the Nobel Prize for Chemistry in 1993. Requirements for PCR include a DNA template, primers, Taq polymerase enzyme, dNTPs, buffer solution and magnesium ions. There are several applications and variations of PCR including quantitative real-time PCR, reverse transcription PCR, and inverse PCR.