An explosion of recent evidence is revealing a new cellular pathway for silencing specific genes at the messenger RNA (mRNA) level that may protect organisms against viruses and genetic damage. This mechanism, which is very widespread--it occurs in organisms ranging from the mold Neurospora to plants, worms, simple vertebrates like the zebrafish, and perhaps even mammals--is apparently triggered when the cell senses some kind of danger, such as a replicating virus or abnormal mRNA. By learning how the cell directs an RNA-cutting enzyme (ribonuclease) to specifically degrade just the mRNAs related to the trigger without affecting other genes, researchers have been able to devise a new method of inactivating specific genes--an ability that should be very useful for studying gene function and might also be used to create genetically modified plants and other organisms.