The ability of double-stranded RNA (dsRNA) to block gene expression is termed RNA interference (RNAi). RNAi has been identified in Drosophila, Caenorhabditis , and other organisms as a way to suppress transposition and to silence endogenous genes. RNAi occurs after transcription and requires the specific degradation of mRNA. Using an in vitro Drosophila lysate system, Zamore et al . identified that dsRNA is cleaved with a periodicity of 21 to 23 nucleotides (nt). RNAi of Renilla reniformis luciferase mRNA was ATP-dependent, but no de novo protein synthesis was required, as demonstrated by the addition of cycloheximide to the lysate system. Radiolabeling within either strand of dsRNA and subsequent incubation with Drosophila lysates led to the comparable appearance of 21- to 23-nt RNAs, suggesting that RNA cleavage occurs through symmetric processing. The authors also showed that following the cleavage of dsRNA into 21- to 23-nt fragments, the mRNA target is also cleaved into 21- to 23-nt fragments that are determined by the regions of dsRNA created by annealing of antisense fragments to their ribonucleotide complements in the mRNA. Cleavage of the mRNA occurs with greatest frequency at uracil residues, but whether this is part of a strict recognition code is unknown. Zamore, P.D., Tuschl, T., Sharp, P.A., and Bartel, D.P. (2000) RNAi: Double-stranded RNA directs the ATP-dependent cleavage of mRNA at 21 to 23 nucleotide intervals. Cell 101 : 25-33. [Online Journal]