volatile amines. except for taar1, all functional taars in humans are expressed in the olfactory epithelium. a third class of olfactory receptors known as vomeronasal receptors has also been identified; vomeronasal receptors putatively function as pheromone receptors. as with many other gpcrs, there is still a lack of experimental structures at atomic level for olfactory receptors and structural information is based on homology modeling methods. in 2023 the structure of or51e2 was found, the first elucidation of the structure of any olfactory receptor. the limited functional expression of olfactory receptors in heterologous systems, however, has greatly hampered attempts to deorphanize them (analyze the response profiles of single olfactory receptors). this was first completed by genetically engineered receptor, or-i7 to characterize the “odor space” of a population of native aldehyde receptors.