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Biogenesis of miRNA and its regulatory mechanisms: miRNAs are encoded from either intergenic regions or introns and sometimes more than one encoded miRNA shares the same transcript (polycistronic). The primary miRNA (pri-miRNA), an approximately 200 kilobases long, capped and polyadenylated transcript, is transcribed by RNA polymerase III (or II) from any of the above mentioned encoding regions of miRNA. The pri-miRNA from intergenic and intronic regions is shown in the upper panel and from polycistronic region is shown in the lower panel of pri-miRNA. Drosha (an RNase III enzyme) along with Pasha (a double strand RNA binding protein) processes pri-miRNA into approximately 70 nucleotide pre-miRNA by cleaving the 5′ cap and polyadenylated tail. The pre-miRNA is transported from the nucleus to the cytoplasm by RAN-GTP exportin 5. Dicer (another RNase III endonuclease) cleaves pre-miRNA into double stranded transient duplex miRNA (22 nucleotides). The double stranded miRNA dissociates into single strands, and only one strand is retained as mature miRNA and it binds to RNA induced silencing complex (RISC). The miRNA-RISC complex regulates the dose of a particular protein by either degrading the complimentary mRNA by using RNAi mechanism or by inhibiting the translational machinery of protein.