RNA interference is a form of post-transcriptional gene silencing (PTGS) mediated by double-stranded RNA (dsRNA).

Specifically, short (21-23nt) intefering RNA (siRNA) oligonucleotides or "guide RNA" molecules are produced by the intracellular cleavage of longer dsRNA via an dsRNaseII enzyme homolog called Dicer.

These siRNA molecules can mediate the destruction of messenger RNA (mRNA) and thus silence further transcirption from a specific gene. The siRNA molecules can be designed to complement part of the sequence of an mRNA and introduced into cells as DNA constructs capable of producing dsRNA, long dsRNA molecules or siRNA. The target mRNAs could be from an active cancer-causing gene or the result of virus replication. Destroying the mRNA halts protein production.

When researchers looked more closely at Drosophila they found that siRNAs associate with the RNA-induced silencing complex (RISC) consisting of a number of enzymes indluding a helicase which unwinds the dsRNA. The RISC is guided to complementary transcripts by the siRNA and then proceeds to cleave the mRNA.

In humans, a different model has been proposed whereby an RNA-dependent RNA polymerase (RDRP) and a helicase combine with the siRNA to guide the complex to the mRNA. The unwound siRNA strands act as primers for the creation of complemntary strands via RDRP activity. New dsRNA is then created and is subsequently attacked by Dicer. In this way more siRNA is made and the pricess of gene silencing is amplified.