How does RNAi work? Genetic and biochemical data indicate a possible two-step mechanism for RNA interference (RNAi): an initiation step and an effector step. a | In the first step, input double-stranded (ds) RNA is processed into 21–23-nucleotide 'guide sequences'. Whether they are single- or double-stranded remains an open question. An RNA amplification step (shaded box) has been suggested on the basis of the unusual properties of the interference phenomenon in whole animals, but this has not been reproduced definitively in vitro. b | The guide RNAs are incorporated into a nuclease complex, called the RNA-induced silencing complex (RISC), which acts in the second effector step to destroy mRNAs that are recognized by the guide RNAs through base-pairing interactions. We also suggest the incorporation of an active mechanism to search for homologous mRNAs. (Endo, endonucleolytic nuclease; exo, exonucleolytic nuclease; recA, homology-searching activity related to E. coli recA.)