mRNA silencing by RNA interference (RNAi) is a regulatory cellular mechanism which was recently elucidated. In 1990, two research groups observed that the insertion of several extra copies of a pigment gene into petunias did not result in more intensely colored plants but rather to a partial or total loss of pigment production. It was determined later that the reduced pigment synthesis was due to a posttranscriptional down-regulation of the mRNA transcribed from the pigment gene. This type of gene silencing was also observed in worms, fungi, fruit flies, and mammals. It is now clear that RNAi is a fundamental regulatory mechanism which evolved more than a billion years ago.