Figure 1 : MicroRNA (miRNA) biogenesis and mechanisms of action. miRNAs are transcribed in one or multiple hairpin structures in the nucleus by RNA polymerase II as large primary transcripts (pri-miRNAs). The pri-miRNAs are cleaved in the nucleus into shorter single hairpin loop called precursor miRNAs (re-miRNAs) by the enzyme Drosha. The pre-miRNAs are exported out into the cytoplasm by Exportin 5. Once in the cytoplasm, the pre-miRNAs are further processed by the enzyme Dicer which removes the hairpin loop and forms the mature double-stranded miRNA. The two strands then unwind and incorporate into the RNA-Induced Silencing Complex (RISC), resulting in the mRNA cleavage or translational repression followed by downregulation of protein expression.