 Mature miRNAs are structurally similar to siRNAs produced from exogenous dsRNA, but before reaching maturity, miRNAs must first undergo extensive post-transcriptional modification.A miRNA is expressed from a much longer RNA-coding gene as a primary transcript known as a "pri-miRNA" which is processed, in the cell nucleus, to a 70-nucleotide stem-loop structure called a "pre-miRNA" by the microprocessor complex.This complex consists of an RNase III enzyme called Drosha and a dsRNA-binding protein DGCR8.