MicroRNA biogenesis pathway. miRNAs are transcribed by RNA polymerase (Pol) II or III into primary (pri)-miRNAs, which are processed by Drosha/DGCR8 (DiGeorge syndrome critical region gene 8) into precursor (pre)-miRNAs. The mature miRNA strand is highlighted in red. The pre-miRNA is transported from the nucleus into the cytoplasm by Exportin 5, where it is then processed by Dicer/TRBP (Dicer-TAR RNA binding protein) into a miRNA duplex. The duplex is unwound by a helicase and the mature strand (red) is incorporated into the RNA-induced silencing complex (RISC). Depending on miRNA complementarity to a target mRNA, the RISC mediates down-regulation of gene expression by either translational repression or mRNA degradation. Ago, Argonaute; PACT, protein activator of PKR; P-body, processing body.