FIGURE 1.

Schematic of miRNA biogenesis. The primary transcripts of miRNA genes (pri‐miRNAs) are transcribed by RNA polymerase II (pol II/III). The initiation step is mediated by the Drosha‐DGCR8 complex. The product of this nuclear processing step is an approximately 70‐nt precursor miRNA (pre‐miRNA). Then, pre‐miRNAs are transported from the nucleus to the cytoplasm through nuclear export factor exportin‐5. Once in the cytoplasm, pre‐miRNAs are recognized and processing step to produce miRNA duplexes by another RNaseIII, Dicer.Then, the duplexes are separated. One strand is usually selected as the mature miRNA and is loaded onto the RISC. If perfect complementarity with the 3′‐UTR of the target mRNAs, the target is cleaved and degraded, if not, the target is not cleaved, but translation is inhibited. In a few cases, miRNA can upregulate the transcription of the target mRNA