RNA interference (RNAi) is a homology-dependent gene silencing technology in which small interfering
RNAs (siRNAs) direct RNA cleavage or DNA methylation. After transcription of an RNAi cassette including inverted repeat
sequences against the target gene and a spacer fragment, the resultant transcript forms a hairpin-like structure. The
stem region of hairpin RNAs is processed into siRNAs. Here we focus on the structural properties of RNAi vectors that
affect the silencing efficiency, and caveats in the evaluation of RNAi phenotype are discussed. Subsequently, several
RNAi applications including simultaneous silencing of multiple gene sequences and specific silencing of a member in the
gene family were discussed. In addition a newly developed RNAi technology, artificial microRNA, is also introduced.