Double-strand RNA (dsRNA) is a signal for gene-specific silencing of expression in a number of organisms. This phenomenon was demonstrated recently in Caenorhabditis elegans when dsRNA was injected into the worm and the corresponding gene products disappeared from both the somatic cells of the organism as well as in its F1 progeny. This RNA interference, RNAi, has been generalized to many genes in C. elegans. ds-RNA can also suppress expression of specific genes in plants, a component of the phenomenon called cosuppression. Two recent reports document dsRNA-mediated interference with expression of specific genes in other organisms. Double-strand RNA produced gene-specific phenotypes in Trypanosoma brucei and, very recently, dsRNA-mediated interference was demonstrated in Drosophila. Thus, the RNAi phenomenon is likely to be a general mechanism for gene regulation and may be critical for many developmental and antiviral processes.