The discovery of mieroRNAs (miRNAs) has advanced our understanding of the fundamental mechanisms of gene regulation.Initially, MicroRNAs act as primary transcripts that are sequentially processed in order to produce the mature active molecules.MiRNA genes are transcribed by RNA polymerase Ⅲ to produce capped and polyadenylated primary transcripts ("pri-microRNAs").These pri-microRNAs are further processed in the nucleus by Drosha and its cofactors Pasha (flies) or DGCR8 (human) to produce~60-nucleotide "pre-microRNAs".Then these are exported to cytoplasm from nucleus by exporting Ran-GTP and are further processed by another RNA Ⅲ enzyme "Dicer" to produce ~22-nucleotide double-stranded RNA duplexes.Finally these duplexes form complexes with the RNA-induced silencing complex (RISC), leading to unwinding of the duplexes to single-stranded matured microRNAs.