6BST:
reference: The crystal structure of Z-DNA with untypically coordinated Ca(2+) ions., Luo Z, Dauter Z, J Biol Inorg Chem. 2017 Dec 21. pii: 10.1007/s00775-017-1526-4. doi:, 10.1007/s00775-017-1526-4. PMID: 29270817

This OCA Structure page
uses Jmol, developed by the Jmol Development Team (documentation).