Small non-coding RNA molecules play important roles in diverse biological processes (Figure 1). In animals, small interfering RNAs (siRNA) can specifically degrade a fully complementary target mRNA in a process called RNA interference (RNAi), whereas endogenous microRNAs (miRNAs) inhibit mRNA translation by associating with sequences in the 3’UTR of target mRNAs. Moreover, small RNA molecules are correlated with transcriptional repression in plants and fungi. Although processing involving siRNA and miRNA result in different outcomes, they share some features: 1) siRNAs and miRNAs are produced from dsRNA molecules cleaved by an RNAse III enzyme called Dicer, 2) these small RNAs then guide a protein complex to their mRNA targets according to their RNA-RNA duplex complementarity. However, little is known about the cellular factors required in each of these pathway and how small non-coding RNAs can regulate diverse aspects of the gene regulation.

Because RNA interference is a sequence-specific process that requires a perfect complementarity between the small RNA molecule and its target gene, it has rapidly become a powerful reverse genetic tool in many organisms. A better comprehension of RNA interference will enforce its utilization as a gene therapy able to target specific genes where their misregulation may cause cancer or other human disease.

Since the discovery of the first microRNAs in the nematode Caenorhabditis elegans (i.e. lin-4 and let-7), it is now clear that these small RNA species are widely found in many cellular organisms and play essential roles in gene regulation. Understanding their functions will lead to a better comprehension of the amazing roles played by RNA molecules in diverse aspects of the cellular maintenance.