To understand how a functional nervous system generates complex behaviors, it is essential to determine the molecular mechanisms that direct the formation of precise connections between afferent axons and their targets during development. In the Drosophila visual system, neuronal circuit assembly depends on intricate bidirectional interactions between photoreceptor axons and target neurons. Anaplastic lymphoma kinase (Alk) and its activating ligand Jelly belly (Jeb) have recently been shown to act as an anterograde signaling system that mediates photoreceptor axon targeting. The aim of this proposal is to elucidate the mechanisms underlying anterograde Jeb/Alk signaling in the visual system. I propose (1) to develop cell-biological and genetic assays to determine whether Jeb acts as a global or local cue; (2) using mosaic analysis and RNA interference approaches, to analyze the potential function of downstream cell adhesion molecules, Dumbfounded/Kirre and its paralog Roughest/IrreC in target neurons; (3) to investigate the role of the interacting factors, Hibris and Sticks and stones; (4) to characterize the function of Frazzled/DCC and a possible link to Jeb/Alk signaling; and (5) using gain-of function and knock-down approaches in specific neuron subtypes, to test whether combinations of these differentially expressed cell-adhesion molecules form a “layer code” for R-cell axons.