The 2006 Nobel Prize in Physiology or Medicine was awarded to two American scientists – Andrew Fire and Craig Mello for the fundamental discoveries in gene expression regulation in eukaryotes. They have shown that the presence of double-stranded RNA in the cell induces specific gene silencing. This pheno- menon was called RNA interference – RNAi. Double-stranded RNA is digested in the cell to short effector RNAs, called siRNAs, which are directly responsible for the selective degradation of target mRNA. The process takes place in the cytoplasm. Double-stranded RNA may appear in the cell as a consequence of viral infection or as a product of transcription of retrotransposones or inverted – repeat sequences. The mechanism is present in almost all eukaryotes, and its primary goal was, most probably, the protection of the cell against invasive forms of nucleic acids. Short RNAs have been also found to be encoded by endogenous, eukaryotic genes. Their mode of action closely resembles that of siRNA molecules. They are involved in the regulation of developmental processes and cell responses to environmental changes.