A. Tanzer, M. Riester, J. Hertel, C. I. Bermudez-Santana, J. Gorodkin,
I. L. Hofacker and P. F. Stadler
In Gustavo Caetano-Anolles, editor, Evolutionary Genomics. Wiley, in press.
MicroRNAs (miRNAs) are an abundant class of small noncoding RNA (ncRNA) genes
which were found in eukaryotes, in particular in metazoans and plants, and in
their viruses. MicroRNA research has come a long way, since the first
discoveries of lin-4  and let-7  in Caenorhabditis elegans. The turn
of the century brought the realization that miRNAs form a large new class of
ncRNAs [101, 97, 95] that provide a ubiquitous and powerful mechanism for
RNA-mediated control of gene expression. The miRBase , a comprehensive
database collecting published miRNAs as well as assigning unique names  to
novel ones, started with only 218 sequences (v1.0, December 2002) and now lists
6396 entries in the current version 11.0 (April 2008). Today (08/05/2008) there
are 3233 publications about miRNAs in PubMed of which 755 are reviews. These
numbers might ilustrate the impact of this field of research on our
understanding of the infomation encoded by the fast majority of genomic
sequences and transcribed units.
MicroRNAs were the first small regulatory RNAs found in animals, but turned out
not to be the only ones. During the past few years, a variety of additional
classes were detected, many of which share functional properties and processing
machinery. In the following section we will address those similarities as well
as differences by outlining biogenesis and function.