4EIY:
reference: Structural basis for allosteric regulation of GPCRs by sodium ions., Liu W, Chun E, Thompson AA, Chubukov P, Xu F, Katritch V, Han GW, Roth CB, Heitman LH, IJzerman AP, Cherezov V, Stevens RC, Science. 2012 Jul 13;337(6091):232-6. PMID: 22798613

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