Olfaction is an ancient sensory system allowing an organism to detect chemicals in its environment. The first step in odor transduction is mediated by binding odorants to olfactory receptors (ORs) which belong to the heptahelical G-protein-coupled receptor (GPCR) superfamily. Mammalian ORs are disposed in clusters on virtually all chromosomes. They are encoded by the largest multigene family (∼1000 members) in the genome of mammals and Caenorhabditis elegans, whereas Drosophila contains only 60 genes. Each OR specifically recognizes a set of odorous molecules that share common molecular features. In mammals, signal transduces through the G-protein-dependent signal pathway in the olfactory sensory neurons that synapse ultimately in the glomeruli of the olfactory bulb, and is finally processed in higher brain structures. The expression of a given OR conditions neuron and glomerulus choices. To date, the processes which monitor OR expression and axon wiring have emerged but are not completely elucidated.