**4.1. Mechanism of target recognition by miRNA and siRNA**

The target recognition of miRNAs is a multifaceted process due to the presence of various binding sites and differing degrees of complementarity between the miRNA and the target mRNA; that is, to induce a functional effect, miRs need to be partially complementary to their target mRNAs. Complementary pairing between mature miRNA and mRNA takes place at the 3′UTR group on the mRNA and the seed region of the miR.-In contrast, other miR binding sites, such as the 3′ supplementary sites, centered sites and bulged sites are believed to be aberrant. Therefore, since perfect matching is not required, one miR strand can target an extensive number of mRNAs; thus one miR often has multiple targets. For example, due to partial complementary base pairing between the miR and the mRNA, AGO2, which plays a vital role in RNA silencing, miRISC is not stimulated. Alternatively, the mRNA targets themselves, silence miR via degradation by deadenylation or translation repression via exonucleases or decapping. Rarely, a high degree of complementarity between the miR and its mRNA target may produce endonucleolytic cleavage of the mRNA through the activity of the AGO protein. This activity is quite similar to the gene silencing mediated by siRNA [2]. In contrast, the siRNA must be matched fully with its target mRNA. The activation of AGO2 after complementary binding, leads to the cleavage of the phosphodiester bonds of mRNA bases 10 and 11 relative to the guide strand 5′ end. Any mRNA fragments produced as a result of this activity undergo rapid degradation through the actions of exonucleases [2].
