At this time, however, we decided that the evidence for the existence of receptors containing 2 different β subunits, or containing five different subunits (Sieghart and Sperk, 2002) is not strong enough yet for justifying inclusion of such receptor types in any list of possible receptors. The same holds true for receptors containing two different γ, δ, or ε per pentamer, or those containing a γ and a δ subunit. So in summary, in the list of “tentative receptors” we only included 9 different receptors (ρ1, ρ2, ρ3, αβγ1, αβγ3, αβθ, αβε, αβπ, αxαyβγ2).
