Figure 1.

The miRNA or canonical pathway produces pri-miRNA transcripts from miRNA genes by RNA polymerase II or III (RNA pol II/III) followed by Drosha/DGCR8 processing of the pri-miRNA transcripts into pre-miRNAs. Intronic pre-miRNA hairpins of the mirtron or noncanonical pathway, transcribed by RNA polymerase II (RNA pol II), are formed by splicing (spliceosome), debranching, and trimming of short introns (lariat) without Drosha processing. Pre-miRNAs generated by both pathways are exported from the nucleus via exportin-5 (Exp 5), followed by subsequent Dicer/TRBP which generates double-stranded-RNAs called miRNA/miRNA*. Argonaute proteins (Ago) unwind and separate the guide strand (miRNA) and the passenger strand (miRNA*). The RISC (RNA-induced silencing complex) incorporates the mature miRNA and interacts with the 3′-UTR of the target mRNA and regulates gene expression by translation inhibition or mRNA degradation.