MicroRNAs (miRNAs) are short ribonucleic acid (RNA) molecules, 20-24 nucleotides long on average and are found in all eukaryotic cells. miRNAs are post-transcriptional regulators that bind to complementary sequences on target messenger RNA transcripts (mRNAs), usually resulting in translational repression and gene silencing. This is one kind of network regulation, one miRNA can controls several genes' expression. The human genome encodes more than 1000 miRNAs, which may target about 60% of mammalian genes and are abundant in many human cell types.

   The DNA transcript pri-miRNA is processed to pre-microRNA (pre-miRNA) in the nucleus by the complex of the nuclease Drosha and the double-stranded RNA binding protein Pasha producing miRNA which is the usual effector of RNA interference.This is then exported from the nucleus bound to Exportin 5. Recent researches have revealed that many transcripts are non protein-coding RNA, including several snoRNAs and miRNAs.