However, microRNAs are not the only type of ncRNA. Aside from the well-known tRNAs, snRNAs and rRNAs involved in protein synthesis, there are snoRNAs involved in RNA modification, rasiRNAs that direct heterochromatin formation and Piwi-interacting piRNAs. In fact, there is a large and growing taxonomy of ncRNA genes ranging from the tiny miRNA at 21 nucleotides to the X-inactivating XIST in humans at 17,000 nucleotides and the massive Air in mice at 108,000 nucleotides long (Satterlee, Barbee, Jin, Krichevsky, Salama et al., 2007). The collected families of ncRNA (607 as of October 2007) are available at Rfam, the RNA families database (www.sanger.ac.uk/Software/Rfam/) (Griffiths-Jones, Moxon, Marshall, Khanna, Eddy et al., 2005). Several ncRNAs have been found to be enriched in brain tissue and have been shown to be involved in brain morphogenesis, neuronal cell fate and differentiation, transcription of neuron-specific genes and possibly even learning and memory (Vo, Klein, Varlamova, Keller, Yamamoto et al., 2005).
