RNA interference (RNAi) is a gene regulatory mechanism that limits transcript levels by either suppressing transcription (transcriptional gene silencing [TGS]) or activating sequence-specific RNA degradation (posttranscriptional gene silencing [PTGS]/RNAi). While TGS and PTGS are mechanistically connected, PTGS is a rapidly expanding field. RNAi was first observed in plants, but later found in various eukaryotic organisms, including fungi and animals. Three forms of RNAi—cosuppression in plants, quelling in fungi, and RNAi in animals—have been described. Recent studies have revealed additional facets of RNAi, such as micro-RNA formation and heterochromatinization. In plants, RNAi is initiated by double-stranded RNA (dsRNA), which cleaves inducer molecules into smaller pieces and eventually degrades target mRNA. This process is involved in protecting the genome against viruses and transposons and in regulating developmental programs. The loss of cytosolic mRNA is not associated with reduced transcription, as shown by run-on transcription tests. DNA expressing the target mRNA may also undergo methylation as a by-product of the degradation process. In plants, the expression of transgenes can lead to the formation of dsRNA, which initiates PTGS. For example, in cosuppressed petunia plants, chsA mRNA forms a partial duplex due to self-complementarity. Similarly, in tomato plants, the introduction of a transgene with an inverted repeat in the 5' untranslated region led to cosuppression of the endogenous acc gene. The production of dsRNA is required to initiate PTGS in plants. Transgenic plants with strongly transcribing transgenes in both sense and antisense orientations show strong PTGS features and can silence endogenous, viral, or foreign genes. In animals, RNAi was first demonstrated in C. elegans by Fire et al., who showed that dsRNA introduced into the body of C. elegans could induce gene silencing. This process was later shown to occur in Drosophila and other invertebrates and vertebrates. RNAi can be initiated by sense or antisense transgenes, and biochemical evidence suggests similar mechanisms operate in both cases. RNAi can spread systemically in worms and through the germ line to several generations. In virus-infected plants, homology-driven RNA degradation occurs during viral genome replication. Viruses can be the source or target of silencing. PTGS mediated by viruses can occur with RNA or DNA viruses. Virus-induced gene silencing (VIGS) has been used to knock down host gene expression. For example, in Brassica napus, inoculation with cauliflower mosaic virus led to systemic viral recovery, with the plant showing resistance to challenge inoculum. VIGS has also been used in Nicotiana clevelandii to study gene function. The key insight in PTGS was the identification of small interfering RNAs (RNA interference (RNAi) is a gene regulatory mechanism that limits transcript levels by either suppressing transcription (transcriptional gene silencing [TGS]) or activating sequence-specific RNA degradation (posttranscriptional gene silencing [PTGS]/RNAi). While TGS and PTGS are mechanistically connected, PTGS is a rapidly expanding field. RNAi was first observed in plants, but later found in various eukaryotic organisms, including fungi and animals. Three forms of RNAi—cosuppression in plants, quelling in fungi, and RNAi in animals—have been described. Recent studies have revealed additional facets of RNAi, such as micro-RNA formation and heterochromatinization. In plants, RNAi is initiated by double-stranded RNA (dsRNA), which cleaves inducer molecules into smaller pieces and eventually degrades target mRNA. This process is involved in protecting the genome against viruses and transposons and in regulating developmental programs. The loss of cytosolic mRNA is not associated with reduced transcription, as shown by run-on transcription tests. DNA expressing the target mRNA may also undergo methylation as a by-product of the degradation process. In plants, the expression of transgenes can lead to the formation of dsRNA, which initiates PTGS. For example, in cosuppressed petunia plants, chsA mRNA forms a partial duplex due to self-complementarity. Similarly, in tomato plants, the introduction of a transgene with an inverted repeat in the 5' untranslated region led to cosuppression of the endogenous acc gene. The production of dsRNA is required to initiate PTGS in plants. Transgenic plants with strongly transcribing transgenes in both sense and antisense orientations show strong PTGS features and can silence endogenous, viral, or foreign genes. In animals, RNAi was first demonstrated in C. elegans by Fire et al., who showed that dsRNA introduced into the body of C. elegans could induce gene silencing. This process was later shown to occur in Drosophila and other invertebrates and vertebrates. RNAi can be initiated by sense or antisense transgenes, and biochemical evidence suggests similar mechanisms operate in both cases. RNAi can spread systemically in worms and through the germ line to several generations. In virus-infected plants, homology-driven RNA degradation occurs during viral genome replication. Viruses can be the source or target of silencing. PTGS mediated by viruses can occur with RNA or DNA viruses. Virus-induced gene silencing (VIGS) has been used to knock down host gene expression. For example, in Brassica napus, inoculation with cauliflower mosaic virus led to systemic viral recovery, with the plant showing resistance to challenge inoculum. VIGS has also been used in Nicotiana clevelandii to study gene function. The key insight in PTGS was the identification of small interfering RNAs (