Vector-based delivery of siRNAs: in vitro and in vivo challenges.

RNA interference (RNAi) induced by small interfering RNAs (siRNAs) has recently become a powerful tool to knock-down gene expression in a sequence-specific manner. In addition to chemically made siRNAs, stable expression of siRNA in the form of short hairpin RNAs (shRNAs) expressed from an RNA polymerase III (pol III) promoter is now widely used approach for the application of RNAi in mammalian cells. However, long-term suppression using constitutive promoters can be problematic and emerging evidence indicates that siRNAs can cause several side effects in human cells. Here we review the recent advances in developing controllable expression vectors in order to accelerate the therapeutic applications of RNAi.