Odorant receptor genes comprise the largest known family of G-protein-coupled receptors in vertebrates. These receptor genes are tightly clustered in the genomes of every vertebrate organism investigated, including zebrafish, mice and humans, and they appear to have expanded and duplicated throughout evolution. In a mechanism that has yet to be elucidated, each olfactory neuron expresses a single receptor gene. This highly restricted expression pattern underlies the ability to distinguish between a wide variety of odorants. Here, we address the evolutionary expansion of odorant receptor genes and the role genomic organization of these genes might have in their tightly regulated expression.