Much has been learned about olfactory receptors since then. Each olfactory neuron is dispersed randomly throughout the olfactory epithelium , and expresses only one OR . However, the receptors are highly promiscuous: each OR responds to several different olfactant molecules, and the same olfactant may stimulate several ORs. It is the combination of activated ORs that encodes a particular smell; its combinatorial nature means that humans may be able to sense up to 10 12 different odors . But much, too, remains unclear. For one thing, olfactory receptors are difficult to isolate; the first experimental structure of a human OR, OR51E2, was not identified until 2023 . The details of the mechanism by which olfactants bind to ORs are also unclear. While makes clear that the usual explanation for protein-cofactor binding, a protein binding pocket accepting small molecules based on their shape, is an important component of olfactant recognition, it does not offer a complete explanation. Compounds with very similar structures may not bind to the same OR, and ones with very different structures may do so. What is more, olfactants with only slightly different structures often have vastly different odors; smells are sometimes-but not always-associated with particular functional groups .