$RNAi$ takes place in all eukaryotic organisms as a method of cellular defense. This method involves silencing of a specific $mRNA$ due to a complementary $dsRNA$ molecule that binds to and prevents translation of the $mRNA$ (silencing). The source of this complementary $RNA$ could be from an infection by viruses having $RNA$ genomes or mobile genetic elements (transposons) that replicate via an RNA intermediate.