Abstract: A method for reducing expression of a target gene in a mammalian cell is provided. The method comprises (a) introducing siRNA into a first mammalian cell such that expression of the target gene is reduced in the first mammalian cell, and (b) exposing a second mammalian cell which does not contain siRNA that reduces expression of the target gene to the siRNA-containing first mammalian cell, thereby resulting in reduced expression of the target gene in the second mammalian cell. An additional aspect provides a method of reducing expression of a target gene in a mammal. Yet another aspect provides a mammalian cell transfected with an expression vector which directs expression of siRNA for a target gene such that expression of the target gene is reduced in the transfected mammalian cell, and the siRNA expression level is sufficient to signal reduction of target gene expression in a mammalian cell not transfected with siRNA for the target gene.