U6 promoter-driven siRNAs with four uridine 3′ overhangs efficiently suppress targeted gene expression in mammalian cells

The first evidence for gene disruption by double-stranded RNA (dsRNA) came from careful analysis in Caenorhabditis elegans(1). This phenomenon, called RNA interference (RNAi), was observed subsequently in various organisms, including plants, nematodes, Drosophila, and protozoans(2-5). Very recently, it has been reported that in mammalian cells, 21- or 22-nucleotide (nt) RNAs with 2-nt 3 overhangs (small inhibitory RNAs, siRNAs) exhibit an RNAi effect(6,7). This is because siRNAs are not recognized by the well-characterized host defense system against viral infections, involving dsRNA-dependent inhibition of protein synthesis. However, the current method for introducing synthetic siRNA into cells by lipofection restricts the range of applications of RNAi as a result of the low transfection efficiencies in some cell types and/or short-term persistence of silencing effects(8). Here, we report a vector-based siRNA expression system that can induce RNAi in mammalian cells. This technical advance for silencing gene expression not only facilitates a wide range of functional analysis of mammalian genes but might also allow therapeutic applications by means of vector-mediated RNAi.