Future directions in this field will also be discussed. MiRNAs were first found in the nematode Caenorhabditis elegans in 1993.1 Since then they have also been described widely in plants and mammals.2 MiRNAs are first transcribed in the nucleus as stem-loop primary miRNA, which are then cleaved into shorter precursor miRNA by Drosha, an RNase III, and its essential Selleck Rapamycin cofactor called DGCR8 (DiGeorge syndrome critical region 8), a double-stranded RNA-binding protein (Fig. 1).3–6 The precursor miRNAs are transported out of the nucleus via Exportin-5 and once in the cytosol are cleaved into their mature form of 20–22 nucleotides by Dicer, another

RNase III.7,8 After cleavage, the miRNA duplex is unwound and the functional strand is loaded onto the RNA-induced silencing complex (RISC) and functions as its guide.9 The mature miRNA guides the RISC complex to a (near) complementary sequence, usually in the 3′ untranslated region (UTR), of a target messenger RNA (mRNA).9 Upon binding, the RISC causes post-transcriptional gene silencing by

either cleaving the target mRNA or by inhibiting its translation, XAV 939 so that miRNAs are usually negative regulators of gene expression.10 In addition to their role in such post-transcriptional repression, miRNAs have now been implicated in transcriptional gene silencing by targeting the promoter region but have also been reported to have a positive effect on transcription.11–13 Each miRNA can potentially regulate the translation

of a large number of different mRNA and each mRNA can check details possess multiple binding sites for a single or for many different miRNA because the specificity of miRNA is mainly determined by Watson-Crick base pairing at the 5′ region of the miRNA. Estimates have suggested that the total number of different miRNA sequences in humans may exceed 1000.14 Computational analysis also predicts that over 60% of human genes are potential targets of miRNAs and that there are a large number of other non-coding RNAs of greater nucleotide length than microRNA, which are also likely to have important functions.15 However, direct experimental evidence defining mRNA targets of miRNA regulation has been reported for only a small number of miRNAs and target mRNAs. Assaying the levels of specific microRNA sequences was initially cumbersome; however, advances in technology now allow detection with a sensitivity and specificity that can enable monitoring in a clinical setting. Originally, RNA blot analyses provided both quantitative and qualitative information about the various forms of a miRNA within a total RNA sample.1,16 As the number of miRNAs in the miRBase registry17 has increased, microarray technology has been adapted to enable the parallel screening of thousands of miRNAs in one sample.18 More recently, real time reverse transcription-polymerase chain reaction has been adapted to enable relative quantification and quantitative analysis of miRNA levels.