A powerful new tool for geneticists is announced in Nature this week in the form of a comprehensive genetic library that can be used to shut off any gene in any tissue of a fruit fly. It works by using a technique called inducible RNAi (RNA interference) to shut off specific genes in specific cells and tissues at specific points during development. This means that, for the first time, researchers will be able to tease out what individual genes do in individual cells without having to "knockout" the gene from the entire animal (which causes all kinds of confusing side effects). Barry Dickson from the IMP in Vienna says his work will inevitably lead to considerable break-throughs given the precision with which researchers will now be able to study the action of specific genes. The work also has direct relevance to humans since there is a significant genetic overlap between many of the systems "of flies and men", meaning that flies can provide clues to the workings of some human genes and tissues.