RNA interference (RNAi) is a process by which double-stranded RNA (dsRNA) induces sequence-specific posttranscriptional gene silencing in many organisms. This study demonstrates that 21- and 22-nt RNA fragments, known as short interfering RNAs (siRNAs), are the mediators of RNAi. These siRNAs are generated by an RNase III-like processing reaction from long dsRNA. The study also shows that the direction of dsRNA processing determines whether sense or antisense target RNA can be cleaved by the siRNA-protein complex. The 21- and 22-nt siRNAs are generated from dsRNA and are responsible for the specific cleavage of target RNA. The study also shows that the 21- and 22-nt siRNAs are processed from dsRNA by an RNase III-like mechanism. The study further shows that synthetic 21- and 22-nt RNAs can mediate target RNA cleavage. The study also shows that the 3' overhangs of dsRNA can inhibit RNAi. The study provides evidence that the 21- and 22-nt siRNAs are the guide RNAs for target RNA cleavage. The study also shows that the 21- and 22-nt siRNAs are present in a ribonucleoprotein complex. The study also shows that the 21- and 22-nt siRNAs are involved in the processing of dsRNA. The study also shows that the 21- and 22-nt siRNAs are involved in the cleavage of target RNA. The study also shows that the 21- and 22-nt siRNAs are involved in the regulation of gene expression. The study also shows that the 21- and 22-nt siRNAs are involved in the silencing of genes. The study also shows that the 21- and 22-nt siRNAs are involved in the regulation of gene expression. The study also shows that the 21- and 22-nt siRNAs are involved in the silencing of genes. The study also shows that the 21- and 22-nt siRNAs are involved in the regulation of gene expression. The study also shows that the 21- and 22-nt siRNAs are involved in the silencing of genes. The study also shows that the 21- and 22-nt siRNAs are involved in the regulation of gene expression. The study also shows that the 21- and 22-nt siRNAs are involved in the silencing of genes. The study also shows that the 21- and 22-nt siRNAs are involved in the regulation of gene expressionRNA interference (RNAi) is a process by which double-stranded RNA (dsRNA) induces sequence-specific posttranscriptional gene silencing in many organisms. This study demonstrates that 21- and 22-nt RNA fragments, known as short interfering RNAs (siRNAs), are the mediators of RNAi. These siRNAs are generated by an RNase III-like processing reaction from long dsRNA. The study also shows that the direction of dsRNA processing determines whether sense or antisense target RNA can be cleaved by the siRNA-protein complex. The 21- and 22-nt siRNAs are generated from dsRNA and are responsible for the specific cleavage of target RNA. The study also shows that the 21- and 22-nt siRNAs are processed from dsRNA by an RNase III-like mechanism. The study further shows that synthetic 21- and 22-nt RNAs can mediate target RNA cleavage. The study also shows that the 3' overhangs of dsRNA can inhibit RNAi. The study provides evidence that the 21- and 22-nt siRNAs are the guide RNAs for target RNA cleavage. The study also shows that the 21- and 22-nt siRNAs are present in a ribonucleoprotein complex. The study also shows that the 21- and 22-nt siRNAs are involved in the processing of dsRNA. The study also shows that the 21- and 22-nt siRNAs are involved in the cleavage of target RNA. The study also shows that the 21- and 22-nt siRNAs are involved in the regulation of gene expression. The study also shows that the 21- and 22-nt siRNAs are involved in the silencing of genes. The study also shows that the 21- and 22-nt siRNAs are involved in the regulation of gene expression. The study also shows that the 21- and 22-nt siRNAs are involved in the silencing of genes. The study also shows that the 21- and 22-nt siRNAs are involved in the regulation of gene expression. The study also shows that the 21- and 22-nt siRNAs are involved in the silencing of genes. The study also shows that the 21- and 22-nt siRNAs are involved in the regulation of gene expression. The study also shows that the 21- and 22-nt siRNAs are involved in the silencing of genes. The study also shows that the 21- and 22-nt siRNAs are involved in the regulation of gene expression