The polymerase chain reaction (PCR) underlies almost all of modern molecular cloning. Using PCR, a defined target sequence that occurs once within a DNA of high complexity and large size—an entire mammalian genome, for example—can be rapidly and selectively amplified in a quasi-exponential chain reaction that generates millions of copies. The reaction is simple to set up, cheap, and undemanding, the only requirement being some knowledge of the nucleotide sequences of the target. In addition to its simplicity, PCR is robust, speedy, flexible, and sensitive.