Schematic representation of biogenesis and function of miRNAs. miRNAs genes, in mono- (as reported in the picture) or polycistronic structures are transcribed by RNA Polymerases II and III as several kb long transcripts (pri-miRNA) characterized by a hairpin structure, and then cleaved by the nuclear RNAse III Drosha. These miRNAs precursors (pre-miRNA) are carried by the Exportin 5 to the cytoplasm where they are subjected to further digestion by the cytoplasmic RNAse III Dicer giving rise to a duplex 19-25 nt microRNA structure. One strand (guided strand) of the duplex (mature miRNA) is incorporated into the RISC complex and carried to the 3'UTR of target mRNAs. If the complementarity between the seed sequence of a miRNA and the 3'UTR mRNA sequence is 100% the mRNA is degraded while suppression of translation is obtained for lower degrees of complementarities.