The central dogma of molecular biology deals with the detailed residue-by-residue transfer of sequential information. It states that such information cannot be transferred from protein to either protein or nucleic acid. The irreversible flow of information is from DNA to RNA to protein; DNA is transcribed into messenger RNA (mRNA) and subsequently translated into protein. However, in recent years it has become clear that additional genetic information exists in the human genome. Non-protein coding RNA (ncRNA) refers to mRNA that is transcribed from DNA but is not translated into protein. These sequences, once thought of as “junk DNA” – portions of the DNA sequence of the genome that don’t have a function – are being found to have crucial roles in human development, physiology and disease. Indeed, recent studies suggest that there are thousands of ncRNAs in the human genome [1-2].
Non-coding RNAs include a class of molecules called microRNAs (miRNAs or miRs). MicroRNAs are highly expressed in normal tissues and are being found to have critical roles in gene regulatory processes during cellular development and differentiation. MicroRNAs are small ncRNAs ~21-nucleotides long that regulate gene expression at the post-transcriptional level. MicroRNAs function by binding target mRNA molecules and either inhibiting translation into protein or targeting them for degradation. Abnormal microRNA expression has been linked to many human diseases, including schizophrenia, autism and cancer.