The RNA interference (RNAi) technique has been used to disrupt gene function in a range of model organisms, but its use in mammalian cells has been problematic. RNAi involves the ribonuclease-III-catalysed digestion of specific double-stranded RNA into 21-22 nucleotide small interfering RNA (siRNA) species. In the May 24 Nature, Elbashir et al. report the application of RNAi in cultured mammalian cells (Nature 2001, 411:494-498). They designed 21-nucleotide siRNA duplexes, with symmetric two-nucleotide 3' overhangs, directed against luciferase reporter transgenes. They tested RNAi in a range of mammalian cell lines (COS, NIH3T3, HeLa and 293). The siRNA duplexes could reduce gene expression (although not completely) in a cell-type-specific manner. Elbashir et al. also demonstrated effective silencing of endogenous genes. RNAi techniques are significantly more efficient than current methods, such as antisense RNA. This study paves the way for wide-scale application of RNAi-driven silencing in a range of functional genomics approaches in mammalian systems.