Introduction

MicroRNAs are small noncoding RNAgenes (18–24 nucleotides in length) that have been identified in different organisms from the nematode C. elegans to humans (for reviews see Bartel, 2004; He and Hannon, 2004). Recently it has become more and more evident that microRNAs play important roles in regulating the translation and degradation of mRNAs through base pairing to perfectly (in plants) or partially (in mammals) complementary sites, mainly but not exclusively in the untranslated region (UTR) of the target mRNA (Lagos-Quintana et al., 2001; Lau et al., 2001; Lee and Ambros, 2001). MicroRNAs are initially transcribed by RNA polymerase II (pol II) as long primary transcripts called primary-miRNAs (pri-miRNAs). A double-stranded RNA-specific ribonuclease called Drosha is responsible for the processing of pri-miRNAs into hairpin RNAs of 70–100bp known as pre-miRNAs, which contain a two nucleotide 3′ overhang characteristic of RNase III cleavage products (Cullen, 2004). Pre-miRNAs are transported to the cytoplasm by the nuclear export factor exportin 5. Once in the cytoplasm pre-miRNAs are processed by a second, double-stranded specific ribonuclease III called Dicer in a 18–24 nucleotide duplex. The product of Dicer's cleavage is incorporated into a large protein complex called RISC (RNA-induced silencing complex), which includes as core components the Argonaute proteins (Ago1–4 in humans). One strand of the miRNA duplex remains stably associated with RISC and becomes the mature miRNA. The opposite strand, called passenger strand or miRNA, is discarded through two different mechanisms.