In RNA interference (RNAi), introduced double-stranded RNA (dsRNA) triggers gene-specific silencing by inducing mRNA degradation. Domeier et al. report in the 15 September Science that some of the worm SMG proteins, which are required for nonsense-mediated mRNA decay, are also required for maintenance of RNAi (Science 2000, 289:1928-1930). RNAi is initiated in the smg mutants as normal, but the animals subsequently recover. SMG proteins may be involved in an amplification step of RNAi. The proteins' possible cleavage or dsRNA-unwinding activities could provide templates for an RNA-directed RNA polymerase.