MicroRNAs are a new class of small, non-coding RNAs that control gene expression at the post-transcriptional level (Ambros, 2004; Bartel, 2004). Primary miRNA transcripts, also called pri-miRNA precursors are processed sequentially by two RNase III enzymes, Drosha and Dicer, to yield intermediate ∼80-nt pre-miRNA precursors and final ∼21-nt mature miRNAs. MicroRNAs are incorporated into RNA-induced silencing complex (RISC) where they identify and silence target messenger RNAs (mRNAs) through translational repression or direct cleavage (Wightman et al., 1993; Olsen and Ambros, 1999; Hutvagner and Zamore, 2002; Doench and Sharp, 2004; Zhang et al., 2004).
Cloning efforts and computational predictions have indicated that there are ∼800 miRNA genes in human, which together regulate more than 5300 genes involved in processes including cell proliferation and metabolism, developmental timing, cell death, hematopoiesis, neuron development, human tumorigenesis, and even DNA methylation and chromatin modification (Ambros, 2003; Baehrecke, 2003; Michael et al., 2003; Bao et al., 2004; Bartel, 2004; Chen et al., 2004; Chen and Lodish, 2005; Johnston et al., 2005). Certain miRNAs are expressed ubiquitously, whereas others are expressed in a highly tissue-specific manner. Their expression levels vary greatly among species and tissues, ranging from less than 10 to more than 50,000 copies per cell (Kim et al., 2004). Less abundant miRNAs routinely escape detection with technologies such as cloning, Northern hybridization, and microarray analysis (Krichevsky et al., 2003; Lim et al., 2003; Liu et al., 2004).