Among the five senses, smell is unique in that it is dedicated to discriminating an enormous diversity of stimulating ligands. Olfactory sensory neurons achieve this detection by the expression of a multigene family of seven-transmembrane G protein–coupled receptors (GPCRs) originally identified by Linda Buck and Richard Axel (Buck and Axel 1991). These receptors, termed olfactory receptors (OR, plural ORs), are expressed in the olfactory tissue of all terrestrial vertebrates examined thus far, and have been shown to respond directly to odorant binding. The identification and subsequent study of ORs has provided great insight into the molecular and neuronal organization of the olfactory system. Indeed, in 2004, Buck and Axel were jointly awarded the Nobel Prize in Physiology for their pioneering work in odorant receptor discovery. Most of the current understanding of ORs results from experiments from the mouse model which...