Introduction
MicroRNAs (miRNAs) are an abundant class of short endogenous RNAs that act as post-transcriptional regulators of gene expression by base-pairing with their target mRNAs. To date more than 3500 microRNAs have been annotated in vertebrates, invertebrates and plants according to the miRBase microRNA database release 8.0 in February 2006 (Griffiths-Jones, 2004; Griffiths-Jones et al., 2006), and many miRNAs that correspond to putative genes have also been identified. The miRNAs identified to date represent most likely the tip of the iceberg, and the number of miRNAs might turn out to be very large. Recent bioinformatic predictions combined with array analyses, small RNA cloning and Northern blot validation indicate that the total number of miRNAs in vertebrate genomes is significantly higher than previously estimated and maybe as many as 1000 (Bentwich et al., 2005; Berezikov et al., 2005; Xie et al., 2005). An increasing body of research shows that animal miRNAs play fundamental biological roles in cell growth and apoptosis (Brennecke et al., 2003), hematopoietic lineage differentiation (Chen et al., 2004), life-span regulation (Boehm and Slack, 2005), photoreceptor differentiation (Li and Carthew, 2005), homeobox gene regulation (Yekta et al., 2004; Hornstein et al., 2005), neuronal asymmetry (Johnston and Hobert, 2003), insulin secretion (Poy et al., 2004), brain morphogenesis (Giraldez et al., 2005), muscle proliferation and differentiation (Chan et al., 2005; Kwon et al., 2005; Sokol and Ambros, 2005), cardiogenesis (Zhao et al., 2005) and late embryonic development in vertebrates (Wienholds et al., 2005).