RNA silencing refers to the process by which the expression of one or more genes is downregulated or entirely suppressed by small non-coding RNAs . Reference is also made to the introduction of an antisense RNA molecule on gene expression. RNA silencing is also defined as sequence-specific regulation of gene expression that is triggered by double-stranded RNA (dsRNA). RNA silencing mechanisms are highly conserved in almost all eukaryotes. The most common and best-studied example is RNA interference (RNAi), in which an endogenously expressed microRNA (miRNA) or exogenously derived small interfering RNA (siRNA) induces the degradation of messenger RNA. complementary (mRNA). Other classes of small RNAs have been identified; including piwi-interacting RNA (piRNA) and its subspecies repeat associated small interfering RNAs (rasiRNAs). The three main classes of small RNAs have currently been identified, namely: small interfering RNAs (siRNAs), microRNAs (miRNAs) and piwi-interacting RNAs (piRNAs). Small interfering RNAs (siRNAs) act in the nucleus and cytoplasm and are involved in RNAi. While siRNAs originate from long dsRNA precursors derived from a variety of single-stranded RNA (ssRNA) precursors, such as sense and antisense RNAs. siRNAs can also originate from hairpin RNAs derived from inverted repeat regions. siRNAs can also be derived enzymatically from non-coding RNA precursors. MicroRNAs (miRNAs) act in the cytoplasm and mediate mRNA degradation or translational arrest. However, some plant miRNAs have been shown to act directly to promote DNA methylation. miRNAs originate from hairpin precursors generated by RNaseIII enzymes, namely Drosha and Dicer. miRNAs and siRNAs either form the RNA-induced silencing complex (RISC)...... middle of article...... can interfere with multiple mRNA species, but each with varying efficiencies different depending on the degree of complementarity. Second, miRNAs generally target the 3' non-coding region of RNA transcripts, whereas most scientists design shRNA constructs for mRNA coding regions. Third, and perhaps most complex, miRNAs can be transcribed in clusters. Additionally, these clusters may contain miRNAs that are the same, similar, or distinct from those in other clusters. One group can interfere with a large group of targets, producing a significant change in phenotype. A second group, induced by a distinct signal, might control some of the same targets as well as distinct mRNAs, thereby producing a different phenotype. Add to this the possibility that some miRNAs may interfere with receptors or transcription factors, which in turn impact multiple pathways and one of these has a system of remarkable complexity..