This landmark paper presents the first structural information for a bona fide calcium channel, revealing the calcium ion in its anionic selectivity filter. Atomic assignments for bound ions are supported by anomalous diffraction measurements from calcium, barium and gadolinium. Importantly, this structure was also the first to show a surprising non-domain-swapped arrangement between the S1-S4 and channel domains of a subunit, which has since been seen in other tetrameric channel structures.