So far as we know, ours is the first report of IFN activation in the target cells by shRNAs delivered by lentiviral transduction. A previous report of IFN induction by lentiviral vector-expressed shRNA only examined the IFN generated in the vector-producing cells, which then up-regulated IFN-stimulated genes in the transduced cells (10) . The distinction is important as lentiviral vectors used in a gene-therapy setting will likely be purified and free of any IFN that has been generated during the vector preparation step, but IFN activation in the target cells would pose a more serious concern. Our data suggest the importance of screening shRNAs for IFN induction in the transduced cells in vitro before largescale studies. An HIV reverse transcriptase inhibitor efficiently blocked IFN production by both sh-B971 and sh-PCAF when delivered by transduction, indicating the virion-encapsulated RNA was not able to trigger IFN activation. In this respect, it is interesting to note that positive-stranded RNA viruses, which produce dsRNA intermediates in the cytoplasm during replication (52) (53) (54) (55) , often replicate in membrane enclosed vesicles (56) , This sequestration of viral dsRNA in membranous structures may shield the RNA from the cytoplasmic PRRs and contribute to a successful infection.