It would be great to identify and classify the full complement of genes in any genome, along with the proteins they encode, and their relationship to normal physiology and to disease. There's only one problem — this outlook ignores the potentially vast number of important functions that are carried out by RNA rather than proteins.

50 years ago, RNA was found to be the crucial component of ribosomes and of other RNA–protein complexes required for mRNA processing. Since then, the number of known non-messenger RNAs — so-called non-coding RNAs (ncRNAs) — has been gradually increasing. What is now known about the various 'flavours' of ncRNAs, and what they do, is engagingly reviewed by Sean Eddy on p919.

Interest in RNA has undergone a recent revival, which began when researchers noticed that short, 21–25-nucleotide RNA molecules (microRNAs) were involved in two seemingly unrelated phenomena: controlling the timing of developmental events in the worm and silencing homologous mRNAs (in a process known as RNA interference). A link between the two processes was revealed earlier this year, when it was found that the two pathways share several components, raising suspicions that the cell had other, naturally occurring microRNAs. The quest for such RNAs has proved remarkably fruitful, with three groups last month reporting the identification of more than 90 conserved microRNAs that, like most other ncRNAs, probably have regulatory roles (see In Brief on p913). Eddy suggests that, far from being the “ragged band of relics” of an RNA-only past, ncRNAs belong to a “modern RNA world” in which RNAs have acquired functions for which, by their nature, they are better adapted for than are proteins.

And finally, we'd like to take this opportunity to wish our readers a happy and peaceful holiday season and New Year.