Fig. 1.

MicroRNA (miRNA) biogenesis pathway. miRNA genes are transcribed by RNA Pol II or Pol III to generate pri-miRNAs whose hairpin structures are cleaved by Drosha/DGCR8 to release pre-miRNAs. Mirtrons are short hairpin introns and are spliced and debranched into pre-miRNAs. Pre-miRNAs are exported from the nucleus by Exportin-RanGTP into cytoplasm. Once in the cytoplasm, pre-miRNAs are further processed by Dicer/TRBP to form an ∼22 nt duplex. One strand is then selected to function as a mature miRNA, and the other strand is degraded.