AGO2 is a highly basic protein that contains a PAZ domain and PIWI domain. It is one of the four members of the mammalian Argonaute protein family (AGO1-4). Among the four members, AGO2 is the only member that possesses the slicer endonuclease activity. Argonaute proteins participate in the various steps of microRNA-mediated gene silencing, such as translational repression and mRNA turnover. The important roles of these small, non-coding RNAs on regulating gene expression in higher eukaryotes have been recently identified. The binding of EIF2C2/AGO2 to a short guide RNA such as a microRNA (miRNA) or short interfering RNA (siRNA) forms minimal RNA-induced silencing complex (RISC). When RISC binds to a perfectly complementary mRNA, generally it will result in silencing due to endonucleolytic cleavage of the mRNA specifically by EIF2C2/AGO2. In contrast, the binding of RISC to a partially complementary mRNA results in silencing through inhibition of translation, and this is independent of endonuclease activity. Knockout mice studies revealed the essential function of AGO2 in embryonic development in mice.