MicroRNAs, also called miRNAs, are a class of RNAs characterized by their small size (on average 22 nucleotides - 21 to 24 in general). MiRNAs are single-stranded, non-coding and specific to eukaryotic cells.
MiRNAs are post-transcriptional regulators capable of silencing the expression of a gene. Their pairing with a target message RNA (mRNA) can lead to the inhibition of its translation or to its degradation depending on the degree of complementarity between the miRNA sequence and that of its target mRNA. MiRNAs are thus part of the regulatory pathways for gene expression.
MiRNAs are also capable of directly methylating DNA in order to turn off genes.
To date, more than 2500 mature miRNAs have been discovered in humans, targeting more than 50% of the genome. A single gene can be regulated by several miRNAs and conversely a single miRNA can regulate several genes due to the often partial complementarity.