miRNA regulation of its target depends upon: 1) degree of complementarity between the miRNA and its binding site (the MRE - miRNA Recognition Element), typically located in the 3’ untranslated region (3’UTR) of the target (Kiriakidou et al., 2004;Yekta et al., 2004), and 2) number of MREs (Valencia-Sanchez et al., 2006). In contrast to plants, complementarity is usually partial in mammals (He and Hannon, 2004). The highest likelihood of successful miRNA:MRE binding in mammals is determined by a perfect match between miRNA seed sequence (nt #2 through #7–8) and adenosine-flanked MRE (Lewis et al., 2005). It is believed that a single MRE of high complementarity generally results in mRNA cleavage (Valencia-Sanchez et al., 2006;Yekta et al., 2004), while multiple sites result in translational repression (Doench and Sharp, 2004;Kiriakidou et al., 2004).
