The olfactory receptor gene family in vertebrates has been shown to evolve through genomic events such as gene duplication and gene conversion. Evidence of a role for tandem duplication is provided by the fact that many olfactory receptor genes belonging to the same phylogenetic clade are located in the same gene cluster. To this point, the organization of OR genomic clusters is well conserved between humans and mice, even though the functional OR count is vastly different between these two species. Such "birth-and-death evolution" has brought together segments from several OR genes to generate and degenerate odorant binding site configurations, creating new functional OR genes as well as pseudogenes.