One class of non-coding RNA that has attracted much attention arises from both intronic and intergenic RNA and is called microRNA. These tiny RNAs are usually only 21 base pairs long even though the DNA coding for them can be up to 6,000 base pairs in length (Saini, Griffiths-Jones, & Enright, 2007). Although small, microRNAs play a big role in gene regulation, especially in the development of the nervous system (Kosik, 2006; Cao, Yeo, Muotri, Kuwabara, & Gage, 2006) and especially in primates (Berezikov, Thuemmler, van Laake, Kondova, Bontrop et al., 2006). More than 500 microRNAs have been catalogued to date that regulate protein-coding genes by binding to and thus silencing mRNA (Lim, Lau, Garrett-Engele, Grimson, Schelter et al., 2005). Amazingly, these 500 microRNAs appear to regulate the expression of more than a third of all coding mRNAs (Lewis, Burge, & Bartel, 2005). Moreover, microRNAs are likely to be just the tip of the iceberg of ncRNA effects on gene regulation (Mendes Soares & Valcarcel, 2006). The list of novel mechanisms by which non-coding RNAs regulate gene expression is growing rapidly (Costa, 2005; Huttenhofer, Schattner, & Polacek, 2005; Vasudevan, Tong, & Steitz, 2007).
