TAAR9 is an orphan receptor that is a member of the rhodopsin GPCRs. TAAR9 may function as a receptor for trace amines, which are biogenic amines such as p-tyramine, ß-phenylethylamine, tryptamine and octopamine that are present in very low levels in mammalian tissues. Although some trace amines have clearly defined roles as neurotransmitters in invertebrates, the extent to which they function as true neurotransmitters in vertebrates has remained speculative. Trace amines are likely to be involved in a variety of physiological functions. References: Br J Pharmacol. 2006 Dec; 149(8): 967–978.