MicroRNAs are a family of small, non-coding RNAs that regulate gene expression in a
sequence-specific manner. The two founding members of the microRNA family were
originally identified in Caenorhabditis elegans as genes that were required for the timed
regulation of developmental events. Since then, hundreds of microRNAs have been identified
in almost all metazoan genomes, including worms, flies, plants and mammals. MicroRNAs
have diverse expression patterns and might regulate various developmental and physiological
processes. Their discovery adds a new dimension to our understanding of complex
gene regulatory networks.