Many routes to a micro RNA.

Micro RNAs (miRNAs) are ∼22 nt small RNAs that function as posttranscriptional negative regulators of gene expression. Thousands of these molecules have so far been identified across the metazoan and plant kingdoms. Although most miRNAs share the same biogenesis pathway, numerous noncanonical miRNAs and other small RNAs have also been identified. In this review, we will discuss these alternate small RNA biogenesis pathways as well as recent discoveries of non-miRNA functions of the miRNA biogenesis machinery. This review will focus only on metazoan pathways.