Figure 1: miRNA biogenesis. miRNAs are first transcribed as long primary miRNA transcripts (pri-miRNAs) by RNA polymerase II inside the nucleus. Pri-miRNA contains a stem-loop structure, a cap at 5’-end and a poly-A tail at 3’-end [74]. Then, pri-miRNA is processed by an enzymatic complex called “microprocessor”, to generate molecules 70-100 nt in length (premiRNA) [77]. Pre-miRNA hairpins are recognised by Exportin-5 (Exp- 5) which allows the passage into cytoplasm, where Dicer creates a 22 nt small RNA duplex. In the cytoplasm, Dicer associates with ds-RNA-binding proteins, like transactivation-response element RNA-binding protein (TRBP) and p53-associated cellular protein (PACT) [78,79]. The helicase activity of Dicer unwounds the RNA duplex, one of the strands preferentially bind to Argonaute (Ago), and the effector complex “RNA induced silencing complex” (RISC) is generated [80]. The strand with less stability at the 5’-end is loaded onto RISC, whereas the other strand, the passenger (miRNA*), is released [24].