RNA interference (RNAi) has become a widely used experimental tool for targeted gene silencing. In the August 10 Science, Scott Hammond and colleagues from the Cold Spring Harbor Laboratory report the isolation of an RNAi effector nuclease from Drosophila cells (Science 2001, 293:1146-1150). They performed a biochemical purification of RISC (RNA-induced silencing complex), a 500 kD ribonucleoprotein complex with sequence-specific nuclease activity, from cultured Drosophila S2 cells. Microsequencing of one of the protein components of the complex revealed a sequence homologous to rde-1, a member of the Argonaute gene family involved in RNAi in Caenorhabditis elegans. Hammond et al. named their gene Argonaute2 (AGO2). AGO2-specific antibodies detected a 130 kD protein in the RISC complex. Disruption of the AGO2 gene by RNAi reduced the cells ability to perform gene silencing induced by double-stranded RNA. This study provides a molecular link between genetic and biochemical observations and will aid understanding of the mechanisms of RNAi.