Gene function studies require loss-of-function phenotypes,which can be achieved either by the generation of gene knockout (KO) animal models or by RNA interference (RNAi) methods.Compared with KO approaches,RNAi is more time-and cost-effective.Furthermore,KO technology was mostly applied on mice so far,with only a few studies on other animal models such as rats and pigs,which also limits its application [1,2].Comparatively,RNAi can work on a much wider spectrum,from Caenorhabditis elegans to mammals [3-5].Lentiviral vectors are useful tools for RNAi because of their ability to transduce non-dividing cells and achieve stable and long-term transgene expression.Moreover,they do not induce a cellular immune response.Lentiviral vectors producing shRNAs have been shown to knockdown gene expression both in vitro and in vivo [6-9].The lentiviral particles can be delivered by local injections to take effect in a specific region or tissue,such as brain and epididymis [5,10-12].Also,lentivirus can be systemically delivered.High-titer lentivirus is essential for significant knockdown of the target gene [5,13],and therefore the production process is crucial.