**4. Biogenesis of miRNAs**

The transcription of miRNAs depends on their location within the genome. Most of the miRNA genes are located throughout the genome in introns, exons and intergenic regions with many miRNAs produced from clusters of coexpressed genes. Some miRNA transcription depends on same RNA polymerase II promoters that drive the transcription of mRNAs. miRNA genes located in intronic regions that includes half of known miRNAs genes often depend on the expression of host gene [52, 53]. Some miRNA genes with independent promoters are tran‐ scribed from their own RNA polymerase II promoters. Additionally a small number of miRNAs genes are transcribed by RNA polymerase III. Those miRNAs organized in clusters for example, miRNA-17-92-family, share the same transcriptional regulation and are grouped together in one cluster on a single unprocessed transcript and expressed together [54].
