The nose is a chemical detector. It houses the olfactory sensory neurons (OSNs), which are specialized cells that display on their surface genomically encoded protein receptors that have evolved to respond to airborne odorant molecules. In 1991, these protein receptors, now known as the olfactory receptors (ORs), were identified by Linda Buck and Richard Axel as belonging to a class of well-known proteins, the G-protein-coupled receptors (GPCRs). (GPCRs are dependent on guanosine triphosphate (GTP) for their function; in the nonstimulated state, these receptors are bound to a complex of three different proteins (a heterotrimer), named alpha (α), beta (β), and gamma (γ), which together constitute the G-protein.)