Schematic representation of expression cassettes using RNA polymerase promoters for generation of small-interfering RNAs (siRNA). (A) Tandem-type promoters express sense and antisense strands individually. After transcription, both strands hybridize forming a duplex siRNA. (B) Short hairpin RNAs (shRNA) are expressed as a single transcript separated by a short loop of 4–10 nt of sequence. The transcripts form a hairpin structure that can be processed by Dicer into functional siRNAs. (C) Expression of imperfect duplex hairpin structures that is based on pre-microRNA (pre-miRNA) structures. pre-miRNAs are processed by Dicer into a mature miRNA, which can direct gene silencing.
