During embryo development, genes are dynamically, and very precisely, switched on and off to confer different properties to different cells and build a well-proportioned and healthy animal. Fgf8 is one of the key genes in this process, controlling in particular the growth of the limbs and the formation of the different regions of the brain. Researchers at EMBL have elucidated how Fgf8 in mammal embryos is, itself, controlled by a series of multiple, interdependent regulatory elements.