A widespread class of non-coding, regulatory RNAs has been recently characterized. Because of their short length (21–22 nucleotides) they are called microRNA or miRNA. These miRNAs have been identified in diverse organisms (prokaryote, eukaryote, vertebrates, invertebrates, plants, fungi) and in viruses (Tuschl et al., 1999; Elbashir et al., 2001; Griffiths-Jones, 2004; Berezikov and Plasterk, 2005; Griffiths-Jones et al., 2006). Their apparently ancient function is to regulate specific protein concentration by inhibiting the first step of translation or by inducing specific mRNA degradation by 3′ UTR binding (He and Hannon, 2004; Pillai, 2005; Valencia-Sanchez et al., 2006). Both molecular and bioinformatics tools have been used to identify candidate miRNAs and their target mRNAs. Based on the numbers generated in these studies, it is estimated that vertebrate genomes may contain hundreds of miRNA genes that may regulate stability or translation of approximately one quarter of all mRNAs (Bentwich et al., 2005; Berezikov and Plasterk, 2005; Legendre et al., 2005; Xie et al., 2005).
Disruption of miRNA function often produces aberrations of important processes including organogenesis, and cell diversification, proliferation, and survival (Reinhart et al., 2000; Brennecke et al., 2003; Dostie et al., 2003; Ambros, 2004; Calin et al., 2004; Alvarez-Garcia and Miska, 2005; Giraldez et al., 2005). The miRNA function has also been implicated in regulating stem cell renewal and the onset of certain cancers (Hatfield et al., 2005, Lu et al., 2005). Therefore, miRNAs regulate important processes in animal development, physiology and disease.