The Argonaute protein family, first discovered for its evolutionarily conserved stem cell function, plays a central role in RNA silencing processes as essential components of the RNA-induced silencing complex (RISC). RISC is responsible for the gene silencing phenomenon known as RNA interference (RNAi). Argonaute proteins bind different classes of small non-coding RNAs, including microRNAs (miRNAs), small interfering RNAs (siRNAs) and Piwi-interacting RNAs (piRNAs). Small RNAs guide Argonaute proteins to their specific targets through sequence complementarity (base pairing), which then leads to mRNA cleavage, translation inhibition, and/or the initiation of mRNA decay. The name of this protein family is derived from a mutant phenotype resulting from mutation of AGO1 in Arabidopsis thaliana, which was likened by Bohmert et al. to the appearance of the pelagic octopus Argonauta argo. {{Infobox protein family | Symbol = Piwi | Name = Argonaute Piwi domain | image = 1u04-argonaute.png | width = | caption = An argonaute protein from Pyrococcus furiosus. PDB . PIWI domain is on the right, PAZ domain to the left. | Pfam = PF02171 | Pfam_clan = | InterPro = IPR003165 | SMART = | PROSITE = PS50822 | MEROPS = | SCOP = | TCDB = | OPM family = | OPM protein = | CDD = cd02826 | PDB = }} RNA interference RNA interference (RNAi) is a biological process in which the RNA molecules inhibit gene expression. The method of inhibition is via the destruction of specific mRNA molecules or by simply suppressing the protein translation. The RNA interference has a significant role in defending cells against parasitic nucleotide sequences. In many eukaryotes, including animals, the RNA interference pathway is found, and it is initiated by the enzyme Dicer. Dicer cleaves long double-stranded RNA (dsRNA, often found in viruses and small interfering RNA) molecules into short double stranded fragments of around 20 nucleotide siRNAs. The dsRNA is then separated into two single-stranded RNAs (ssRNA) – the passenger strand and the guide strand. Subsequently, the passenger strand is degraded, while the guide strand is incorporated into the RNA-induced silencing complex (RISC). The most well-studied outcome of the RNAi is post-transcriptional gene silencing, which occurs when the guide strand pairs with a complementary sequence in a messenger RNA molecule and induces cleavage by Argonaute, that lies in the core of RNA-induced silencing complex. Argonaute proteins are the active part of RNA-induced silencing complex, cleaving the target mRNA strand complementary to