RNA interference (RNAi) is a well conserved process that exists in various organisms from Fungi to plants and mammals. This phenomenon was first encountered in plants and fungi where the researchers noticed that over-expression of certain genes unexpectedly resulted in down-regulation of both endogenous gene and the transgene. Later, Fire and Mello groups (1998) demonstrated that introduction of dsRNA into C. elegans specifically directs its cognate mRNA destruction showing when RNA is introduced into the eukaryotic cells in specific form (i.e. double stranded) a process, named as RNAi, is triggered. Researchers have shown that though the RNAi process can be induced exogenically by sending dsRNA into the cells, but the RNAi is, indeed, an endogenous system that involves the coordinated action of several endogenously expressed enzymes and proteins. Activation of the RNAi machinery mostly results in gene expression down-regulation by directing RNA destruction or translation inhibition, a process known as post transcriptional gene silencing (PTGS), but in some cases chromatin modification is involved. RNAi has naturally a fine tuning function on gene expression helpful in varying physiological conditions and also at different developmental stages, but it also acts as a defense mechanism against invading viruses and protects the host genome from transposable elements. However, the great potentials of this system as a tool for functional genomics studies and therapeutic approaches have made this system very appealing to the biologists. So far, this system has successfully been used for functional characterization of genes with unknown function, unravelling gene functional networks and studying host – pathogen interaction aiming at pinpointing drug targets. Moreover, RNAi has passed several clinical trials targeting various diseases and disorders including viral infections, genetic disorders and cancer.

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