MicroRNAs (miRNAs) are ∼22 nt RNAs that direct posttranscriptional repression of mRNA
targets in diverse eukaryotic lineages. In humans and other mammals, these small RNAs
help sculpt the expression of most mRNAs. This article reviews advances in our understanding
of the defining features of metazoan miRNAs and their biogenesis, genomics, and evolution.
It then reviews how metazoan miRNAs are regulated, how they recognize and cause repression
of their targets, and the biological functions of this repression, with a compilation
of knockout phenotypes that shows that important biological functions have been identified
for most of the broadly conserved miRNAs of mammals.