In the mouse olfactory system, each olfactory sensory neuron (OSN) expresses a single type of odorant receptor (OR) out of approximately 1,000 in a monoallelic manner. Furthermore, OSNs expressing the same OR converge their axons to a specific set of glomeruli on the olfactory bulb. These two basic principles are fundamental to the peripheral olfactory system, and are regulated by the expressed OR protein itself. Singular OR gene choice is ensured by the combination of stochastic enhancer-promoter interaction and negative-feedback regulation by OR proteins. In the axonal projection, OR-derived cyclic AMP signals and neuronal activity determine the expression levels of axon guidance/ sorting molecules, and thereby direct glomerular positioning and axon sorting.