From insects to mammals, olfactory receptor neurons (ORNs) expressing the same odorant receptor target their axonal projections to specific glomerular destinations in the antennal lobe/olfactory bulb, creating an odor map in these first olfactory structures of the central nervous system. In the fly, axons of 50 classes of ORNs match precisely with the corresponding dendrites of 50 classes of second order olfactory projection neurons (PNs) to form the 50 glomeruli of the antennal lobe. Olfactory information is thus faithfully delivered from sensory organs in the periphery to higher brain centers, enabling innate and learned olfactory behavior. Thanks to the support of this grant, our studies over the past ten years have made the Drosophila antennal lobe one of the best-understood circuits in terms of the molecular, cellular, and developmental underpinnings of wiring specificity. We have provided a detailed description of the wiring process with single-cell resolution, finding that PN dendrites pre-pattern the antennal lobe prior to ORN axon invasion. We have uncovered the contributions of PN dendrite-dendrite interactions and ORN axon-axon interactions, as well as PN-ORN synaptic partner matching in the wiring process. We have identified several transcription factors and cell surface receptors that instruct wiring specificity in the above cellular contexts. We have also created novel tools to analyze wiring specificity with single-glomerular and single neuron precision. In this renewal, we propose a series of molecular genetic experiments with the aim of identifying the cell surface code that instructs the precise matching between 50 classes of ORNs and PNs. We will study cell surface proteins that regulate ORN axons in choosing their specific trajectories en route to their destined glomeruli, targeting to specific areas of the antennal lobe, and matching with specific postsynaptic partner PNs. An emphasis will be placed on RNAi-based screens to identify cell surface proteins. We will follow up with in-depth mechanistic studies to place their actions in the proper cellular and developmental context. We expect that completion of the proposed experiments in this grant will significantly enrich and expand our understanding of the logic and mechanisms of olfactory circuit assembly. These studies will contribute to our understanding of a central problem in developmental neurobiology: how wiring specificity of neural circuits is achieved during development. By uncovering the molecules and mechanisms inherent to the fly olfactory circuit, we have already provided insight into constructing more complex neural circuits in the mammalian brain, and established links between neural circuit wiring and disorders of the human brain.