Amongst the different classes of ncRNAs, miRNAs are perhaps the best characterized. Since their discovery in C. elegans in 1993 (Lee et al., 1993), hundreds of miRNAs have been identified in different species. Initial studies demonstrated that these short (∼22 nucleotide) RNAs are mainly involved in binding to the 3′ untranslated region (3′UTR) of mRNA transcripts that share complementarity with nucleotides in their so-called seed sequence (nucleotides 2–7). Binding of miRNAs to mRNA transcripts results in post-transcription silencing of the target mRNA via RNA-induced silencing complex (RISC)-induced translational repression or sequestering them for storage or degradation (Bartel, 2004). However, emerging new evidence suggests that the non-seed region of miRNAs may bind to the 5′UTR or the coding sequence of target transcripts and thereby influence translation processes (Orom et al., 2008; Elcheva et al., 2009). To add further complexity, recent reports show that miRNAs can also positively regulate gene expression in some cellular contexts (Vasudevan et al., 2007; Place et al., 2008). Computational sequence analysis predicts that each miRNA can target 10–100s of mRNA transcripts (Esteller, 2011). In addition, each gene transcript can itself be targeted by potentially hundreds of miRNAs.
