 MicroRNAs ( miRNAs ) are <unk> encoded non @-@ coding RNAs that help regulate gene expression , particularly during development . The phenomenon of RNA interference , broadly defined , includes the endogenously induced gene silencing effects of miRNAs as well as silencing triggered by foreign dsRNA . 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 <unk> and a dsRNA @-@ binding protein <unk> . The dsRNA portion of this pre @-@ miRNA is bound and cleaved by Dicer to produce the mature miRNA molecule that can be integrated into the RISC complex ; thus , miRNA and siRNA share the same downstream cellular machinery . First , viral encoded miRNA was described in <unk> . Thereafter , an increasing number of microRNAs have been described in viruses . <unk> is a comprehensive catalogue covering viral microRNA , their targets and anti @-@ viral miRNAs ( see also <unk> resource : http : / / <unk> / servers / <unk> / ) . 
