4MTF:
reference: Structural basis for Ca selectivity of a voltage-gated calcium channel., Tang L, Gamal El-Din TM, Payandeh J, Martinez GQ, Heard TM, Scheuer T, Zheng N, Catterall WA, Nature. 2013 Nov 24. doi: 10.1038/nature12775. PMID: 24270805

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