Fig. 1

miRNA biogenesis and function. Mature miRNAs arise from a multistep process in which they are first transcribed by RNA polymerase II (RNA pol II) as a primary miRNA (pri-miRNA) transcript. After being cleaved in the nucleus by the RNAse III ribonuclease, Drosha, and its cofactor Pasha, the precursor miRNA (pre-miRNA) is exported to the cytoplasm by Exportin 5, where it is cleaved by a second RNAse III ribonuclease, Dicer. This 16- to 29-nucleotide-long miRNA duplex is then unwound to free the mature strand for incorporation into a RNA-induced silencing complex (RISC) and, based on sequence complementarity, directs translational repression or cleavage of its mRNA target by binding to either the 3′- or 5′-untranslated (UTR) regions