MicroRNAs (miRNAs) are a type of small single-stranded non-coding RNA molecules of 18 ~ 24 nucleotide (nt) in length. MiRNAs have been found in many species including plants, animals and some viruses. The conventional biosynthesis pathway consists of four steps: 1) miRNA genes are transcribed by RNA Polymerase II or III to produce pri-miRNAs, 2) pri-miRNAs are recognized and cleaved by DGCR8/Drosha complex to form miRNA precursor (pre-miRNA), 3) pre-miRNA hairpins are exported from the nucleus to cytoplasm mediated by nucleocytoplasmic shuttler Exportin-5, 4) in the cytoplasm, the pre-miRNA hairpins are further recognized and cleaved by enzyme Dicer to generate the mature miRNAs.