Although the first published description of an miRNA occurred more than ten years ago , only recently has the breadth and importance of this class of small, regulatory RNAs been appreciated. Hundreds of unique miRNAs are encoded in animal, plant, and fungal genomes and are expressed in a regulated manner [reviewed in 6]. miRNAs are transcribed as parts of longer RNA molecules  that are processed in the nucleus into hairpin RNAs of 70-100 nucleotides by the dsRNA-specific ribonuclease Drosha  (Figure 1). The hairpin RNAs are transported to the cytoplasm and digested by a second, double-strand specific ribonuclease, Dicer. The resulting 19-23mer miRNA is bound by a complex that is similar to or identical to the RNA-Induced Silencing Complex (RISC), which participates in RNA interference . The complex-bound, single-stranded miRNA binds mRNAs with sequences that are often significantly, though not completely, complementary to the mRNA. In animals, the bound mRNA typically remains intact but is not translated, resulting in reduced expression of the corresponding gene.
Figure 1. miRNA Processing and Activity.
miRNAs as Regulators of Global Gene Expression
Several hundred miRNAs have been
cloned and sequenced from mouse, human, Drosophila,
Caenorhabditis elegans, and Arabidopsis samples.
Estimates suggest that 200-300 unique miRNA genes are present
in the genomes of both humans and mice . The sequences of
many of t