In 1990, it was already discovered that plant transformation with a transgene containing its homologue in the plant nuclear genome is able to promote silencing of both the transgene and the homologous, endogenous gene. The phenomenon was named posttranscriptional gene silencing (PTGS) or co-suppression. The same results were obtained when a transgene was introduced into the nuclear genome of fungus Neurospora crassa. This process was termed quelling. In 1998, RNAi (RNA interference) was discovered in the Caenorhabditis elegans worm. Specific gene silencing occurred after the introduction into the worm of cells of double stranded RNA with sequence complementarity to the endogenous gene. It was shown that RNAi operates at the stage of the mature mRNA in the cytoplasm. dsRNAs are converted into siRNAs (small interfering RNAs) due to the Dicer enzyme activity. siRNAs are incorporated into the RISC (RNA ? induced silencing complex). Active RISC promotes specific mRNA degradation. RNAi/PTGS/quelling processes show many mechanistic similarities, but they also differ in some details. All of them represent an ancient mechanism that probably evolved to protect eukaryotic cells against invasive forms of nucleic acids like viruses, transposons, and others.