The process of polymerase chain reaction (PCR) was first described by Kjell Kleppe and 1968 Nobel laureate H. Gobind Khorana. It allows specific DNA sequences to be amplified (multiplied hugely). Mullis greatly improved the technique, and it became a standard tool in DNA research.
In 1983, Mullis was working for Cetus Corp. as a chemist. That spring, according to Mullis, he was driving his vehicle late one night, when he had the idea to use a pair of primers to bracket the desired DNA sequence and to copy it using DNA polymerase. This would allow a small strand of DNA to be copied almost an infinite number of times. Cetus took Mullis off his usual projects to concentrate on PCR full-time. Mullis succeeded in demonstrating PCR on December 16, 1983.
Cetus paid him a mere $10,000 for the discovery but later sold it to Hoffmann-La Roche, owned by Roche Holding Ltd., for $300 million.