The sense of smell is essential for food detection, prey and predator recognition, reproduction and other intraspecies communication. Several olfactory receptor gene families, both small and large ones, contribute to the detection of odors. We have recently characterized one of these olfactory receptor gene families, the TAAR genes, in lower vertebrates 1 and found a series of physiologically relevant ligands for a receptor from this family that elicit aversive behavior in close correspondence to their effectiveness as ligands. These ligands also activate a sparse subset of olfactory receptor neurons (see figure). Thus we have identified a defined neural circuit that spans from peripheral sensory detection via a single receptor to generation of specific behavior. This is a novum for any vertebrate species and constitutes an excellent basis to study neuronal development and plasticity in a defined neural circuit. Intriguingly, olfactory receptor molecules are not only involved in odor perception, but also appear to serve double duty in axonal pathfinding of olfactory sensory neurons, as has been examined in rodents 2. We suggest to study this phenomenon in the functionally identified neural circuit described above, using zebrafish, Danio rerio, as experimental system. Zebrafish is an established model system for studying olfaction  and has many advantages especially for studying early development.