RNA interference (RNAi) efficiently induces sequence-specific gene silencing in mammalian cells through short interfering RNA (siRNA) of 21-23 nucleotides synthesized in vitro or expressed by DNA-based vector. However, introduction of siRNA into mammalian cells by transfection limits the application of RNAi, especially when it is necessary to generate long-term gene silencing in vivo. Virus vector-mediated RNAi provides an alternative to transfection. In the present study, we investigated such transduction system and showed that retrovirus vector-mediated RNAi can substantially down-regulate expression of mouse adult beta-globin gene in MEL cells. The results suggest that retrovirus vector-delivered RNAi may find its use in functional genomics and in gene therapy.