Biology teaches that genetic information flows DNA → RNA → protein, the central dogma. However, when double-stranded RNA (dsRNA) enters a cell, messenger RNAs with matching sequences are destroyed and the corresponding proteins are no longer produced. This phenomenon is called RNA interference (RNAi). It is believed to have evolved as a defense that detects dsRNA from invading viruses. In their landmark work, Fire and Mello injected synthetic dsRNA into the nematode Caenorhabditis elegans and could silence almost any chosen gene. RNAi shares its core machinery with microRNAs (miRNAs) that normally regulate development and differentiation. Today the technique is applied to crop improvement, drug discovery, and suppression of cancer cell growth, demonstrating clear social benefits. Because genes can be switched off at will, RNAi has become a routine tool in life-science research.

RNA interference begins when long double-stranded RNA is cleaved by the RNase-III enzyme Dicer into ~21–23-nucleotide small interfering RNAs (siRNAs). The siRNA duplex is loaded into the RNA-induced silencing complex (RISC) whose core is an Argonaute protein; the duplex unwinds and the antisense strand guides RISC to complementary mRNA. Perfect pairing triggers the 'slicer' activity of Argonaute, cutting the mRNA, which is then degraded by exonucleases. In nematodes, only a few dsRNA molecules per cell can elicit systemic knock-down, suggesting amplification by RNA-dependent RNA polymerase. In plants and fungi, RNAi partners with heterochromatin formation to restrain transposons and viruses, safeguarding genome stability. Mammalian cells mount an interferon response to long dsRNA, so researchers employ synthetic 21-nt siRNAs or vector-expressed shRNAs. RNAi now underpins functional genomics, disease-model creation, and even approved therapeutics such as Patisiran. Ongoing challenges—off-target effects, delivery, innate immunity—are addressed through refined design algorithms and advanced nanoparticle carriers.

Dicer processively cleaves dsRNA into siRNAs bearing 2-nt 3' overhangs, and terminal phosphorylation is sensed by loading factors such as R2D2 or TRBP. Human AGO2 contains an RNase-H-like catalytic core (D597, D669) within its MID/PIWI domains and cleaves target mRNA 10–11 nt from the guide’s 5' end. RNA-dependent RNA polymerases generate secondary siRNAs in both primer-dependent and de novo modes; in the NRDE pathway or the plant AGO4/Pol IV–V complex, 24-nt siRNAs recruit H3K9 methyltransferases and DNA methyltransferases to enforce transcriptional silencing. GalNAc-conjugated or LNP-encapsulated siRNAs enable liver-specific delivery, leading to FDA-approved drugs such as ALN-TTR02 (Patisiran). Off-target mitigation strategies include tuning thermodynamic asymmetry of the seed region and applying 2'-OMe, 2'-F, or LNA modifications to modulate Argonaute affinity.