Axon guidance is the study of how developing nerves navigate in response to external signals. A remarkably small number of navigational signals have been identified, raising the question of how the complexity of the brain is generated. One solution is that a single ligand can signal in different ways either through the presence of different receptors or through processing that alters receptor binding. Slit is a large secreted protein that typically repels growing axons using Robo receptors. Slit is cleaved into two fragments, Slit-N and Slit-C, which display new biological activities in the nervous system and that in many other tissues. However, the role of the Slit fragments is controversial. The Slit-N fragment contains the Robo binding site and has been shown to be repel axons in in vitro culture systems and so is thought to be the active signaling molecule. Multiple lines of in vivo preliminary evidence in Drosophila suggest that only the full-length (Slit-FL) protein repels axons. This proposal takes an in vivo molecular genetic approach in flies and mice to separate the signaling of Slit-FL from that of the Slit fragments.These two systems will allow us to address the effects of Slit on axon growth, axon branching and regulated adhesion (fasciculation). This work will allow us to determine how a single signal can achieve different biological outcomes. The proposed work has implications for a diverse range of fields, including infectious disease, cancer and nerve regeneration.