Olfaction faces a formidable challenge: to make sense of a world that it has never encountered. To achieve this task, sensory neurons located in the nose express different types of chemoreceptors. These are encoded by the largest gene families present in vertebrates genomes, and include five unrelated groups of GPCRs: odorant, TAAR, V1R, V2R and FPR receptors. The two first families are expressed in the main olfactory system, while the three latter are expressed in the vomeronasal organ.