1XTC:
reference: The three-dimensional crystal structure of cholera toxin., Zhang RG, Scott DL, Westbrook ML, Nance S, Spangler BD, Shipley GG, Westbrook EM, J Mol Biol 1995 Aug 25;251(4):563-73. PMID: 7658473

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