In diverse eukaryotes, dsRNA triggers the destruction of mRNA sharing the same sequence as the dsRNA in the process called RNAi. The guides for sequence-specific degradation of mRNA are 21 nt short interfering RNAs (siRNAs). Synthetic siRNAs can efficiently mediate RNAi, but a drawback of RNAi is its transient nature as a result of the limited availability and stability of synthetic oligonucleotides. Recently, several groups reported the construction of expression plasmid vectors that mediate the production of siRNAs under control of Pol III promoters. These vectors allow the continued expression of siRNAs in the cells resulting in persistent and specific suppression of target genes. The retroviral siRNA expressing system allows for stable inactivation of the genes in primary cells or living organisms.