Figure 1

The canonical pathway of miR processing. The primary miR (pri-miR) are transcripted by either RNA polymerase II or III from independent gene in the nucleus. In the following processing, the microprocessor complex (Drosha-DGCR8) processes pri-miR into a ~60-100-nucleotide precursor hairpin (pre-miR). The resulting pre-miR is exported to the cytoplasm by Exportin-5-Ran-GTP. In the cytoplasm, the RNase III Dicer and TRBP cleave the pre-miR into ~22-nucleotide miR/miR* duplex. One strand termed as guide strand, further representing a mature miR, the miR* termed as passenger strand, which undergoes degradation rapidly. Mature miR is incorporated into a miRISC and base-paired to its target mRNAs for mRNA degradation or translational repression.