Since the discovery of the first microRNA (miRNA) family member lin-4
in Caenorhabditis elegans by Lee et al. and RNA interference (RNAi) by Andrew Fire and his colleagues in the 1990s, the new field of regulatory non-coding RNAs has enormously gained momentum and importance. Small regulatory RNAs comprise small interfering RNAs (siRNAs), miRNAs and Piwi-associated small RNAs (piRNAs). Generated from double-stranded RNAs (dsRNAs), siRNAs trigger sequence-specific mRNA decay also known as RNA interference (RNAi). miRNAs in association with Argonaute (AGO ) and GW182 proteins, forming the RNA-induced silencing complex (RISC), mediate fine tuning of gene expression and are involved in various biological key processes. An estimate of 500-1,000 miRNA genes exist in vertebrates and plants and about 100 in invertebrates. Each miRNA is predicted to target hundreds of mRNAs thus influencing key regulatory mechanisms of the cell. Consequently, deregulated miRNA expression has been suggested to contribute to the initiation and progression of human cancer and other diseases. piRNAs associated with Piwi proteins protect the animal germline from mobile genetic elements, thereby acting as a small RNA-based immune system.