The 2006 Nobel Prize in Physiology or Medicine goes to Andrew Z. Fire and Craig C. Mello "for their discovery of RNA interference - gene silencing by double-stranded RNA". This is the now quite well-known phenomenon of RNA interference (RNAi), in which double-stranded RNA can interact with complementary messenger RNA to block the expression of genes. This discovery was important as it showed how RNA can carry out a specific cellular function without having to be translated into a protein. Since then, there have been many other types of RNAs found to act as gene regulators, including the microRNAs (miRNAs) mentioned by Coffee Mug recently. Some of the protein players involved in RNAi, such as the enzyme Dicer, have been identified, but the mechanisms of action of all these RNAs, especially the more recently discovered types, are still quite uncertain.