The fruit fly Drosophila is powerful for studying development and disease and there are many tools to genetically modify its cells. One tool, the Gal4/UAS system, has been a mainstay of Drosophila genetics for twenty-five years. But it only functions effectively in non-reproductive cells, not in egg-producing cells. It has not been known why. Scientists have discovered why and developed a new tool that can work in both cell types.