[Regulations through small RNAs]

For a long time RNA molecules have been viewed as simple intermediates between DNA and proteins as conveyed by the name "messenger RNA". However, the similarity between RNA and DNA creates multiple opportunities for regulatory interactions between genes and their transcripts. Over the last ten years a large body of studies in different eukaryotes has shown that indeed RNA molecules play major roles in the control of gene expression. The first pathway to be analyzed has been the control of translation and degradation of messenger RNA by RNA interference and the related processes. This has led to the realization that regulatory RNA molecules have two specific features : a size around twenty nucleotides and the fact that at one point they have been double stranded. The field of action of small regulatory RNA is however much broader and also covers several levels of genomic organisation such as chromatin compaction, gene rearrangement and spatial organisation of the nucleus. Thus small regulatory RNA are involved at all levels of gene expression and are central to cellular regulations.