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.