the target cleavage. The active site with aspartate – aspartate – glutamate triad harbors a divalent metal ion, necessary for the catalysis. Family members of AGO that lost this conserved feature during evolution will lack the cleavage activity. In human AGO, the PIWI motif also mediates protein-protein interaction at the PIWI box, where it binds to Dicer at one of the RNase III domain. At the interface of PIWI and Mid domains sits the 5′ phosphate of a siRNA, miRNA or piRNA, which is found essential in the functionality. Within Mid lies a MC motif, a homologue structure proposed to mimic the cap-binding structure motif found in eIF4E. It was later found that the MC motif is not involved in mRNA cap binding Family members In humans, there are eight AGO family members, some of which are investigated intensively. However, even though AGO1–4 are capable of loading miRNA, endonuclease activity and thus RNAi-dependent gene silencing exclusively belongs to AGO2. Considering the sequence conservation of PAZ and PIWI domains across the family, the uniqueness of AGO2 is presumed to arise from either the N-terminus or the spacing region linking PAZ and PIWI motifs. Several AGO family members in plants also attract study. AGO1 is involved in miRNA related RNA degradation, and plays a central role in morphogenesis. In some organisms, it is strictly required for epigenetic silencing. It is regulated by miRNA itself. AGO4 does not involve in RNAi directed RNA degradation, but in DNA methylation and other epigenetic regulation, through small RNA (smRNA) pathway. AGO10 is involved in plant development. AGO7 has a function distinct from AGO 1 and 10, and is not found in gene silencing induced by transgenes. Instead, it is related to developmental timing in plants. Disease and therapeutic tools Argonaute proteins were reported to be associated with cancers. For the diseases that are involved with selective or elevated expression of particular identified genes, such as pancreatic cancer, the high sequence specificity of RNA interference might make it suitable to be a suitable treatment, particularly appropriate for combating cancers associated with mutated endogenous gene sequences. It has been reported several tiny non-coding RNAs(microRNAs) are related with human cancers, like miR-15a and miR-16a are frequently deleted and/or down-regulated in patients. Even though the biological functions of miRNAs are not fully understood, the roles for miRNAs in the coordination of cell proliferation and cell death during development and metabolism