Current evidence suggests that only 11 subtypes can be listed as conclusively identified, and these are reasonably abundant. Also listed are several subtypes for which the evidence is strong but not conclusive (6 subtypes). Finally, subtypes are included containing one of each of the minor subunits, evidence for whose native existence is tentative (another 8, plus one subtype with two kinds of α subunit, for a total of 9, and a grand total of 26). Note that one (ρ) overlaps with list A. An additional similar number of subtypes which are relatively rare, but are likely to exist, are not listed at this time, but the list will continue to grow as more information becomes available. Each subtype could play a significant role in the cells in which they occur. It should be noted that even these minor GABAA-R subtypes are present in amounts comparable to, or greater than, receptor subtypes for other accepted brain neurotransmitters other than glutamate; that is the biogenic amines and acetylcholine. Clearly, the heteromeric ligand-gated ion channels offer much greater heterogeneity than other known receptor subtypes.
