For quite some time, the main focus in molecular biology was on DNA, being the carrier of the genetic code, with RNA being viewed merely as an intermediary player. However, lately it has become clear that RNA plays a much more prominent role and is a key player in the cell's regulatory mechanism.

Various new types of RNA have been discovered, which has provided a further boost to RNA research. As a result, research into small regulatory RNA molecules and in particular microRNA (miRNA) has experienced an exponential gain in attention. The realization that RNA molecules play an essential role in gene regulation has fuelled research in this area, leading to a steep increase in the amount of papers published on regulatory RNA molecules.

The award of the 2006 Nobel Prize in Physiology or Medicine to Andrew Fire and Craig Mello for their discovery of RNA interference - gene silencing by double-stranded RNA - is a clear sign of the importance of this field. MicroRNA (miRNA) are naturally occurring, non-coding strands of RNA that trigger the RNA interference pathway. They result from an extensive processing route in which a long RNA transcript, folded in a hairpin structure, is cleaved into short strands of approximately 22 nucleotides - the miRNA molecules.

MiRNAs regulate gene expression by controlling the efficiency of messenger RNA (mRNA) translation - the process of translating genetic information into proteins. The miRNA binds to its target sequence in mRNA transcripts, which leads to translational repression or mRNA degradation. As a result, the production of protein encoded by that particular mRNA sequence is inhibited.

So far, approximately 550 human miRNA sequences have been documented. Of many it is not clear what genes they target. However, some miRNAs have been reported to regulate the expression of genes involved in differentiation and cell growth, tumor suppressors and oncogenes. Each miRNA may regulate the expression of multiple genes. miRNAs are now recognized as one of the key regulators of gene expression, involved in almost every aspect of a cell life from cell differentiation to apoptosis, although specific functions have been elucidated only for a handful of miRNAs.

Check the references below for some relevant research papers and reviews on miRNAs