The modified honeybee toxin, TPNQ (cyan), blocks wt-Kir1.1 by binding to the outer mouth of the channel at F129-Kir1.1b (yellow side chains). TPNQ also blocks the mutant channel, R128Y-Kir1.1b, but with a 100 fold higher KD, consistent with the Tyr side-chain of R128Y (cyan) impeding close approach of the toxin. picture was drawn with PYMOL from our homology model, based on Kir2.2,13 (see Methods) and the known structure of TpNQ.16