3IGC:
reference: Insights from the structure of a smallpox virus topoisomerase-DNA transition state mimic., Perry K, Hwang Y, Bushman FD, Van Duyne GD, Structure. 2010 Jan 13;18(1):127-37. PMID: 20152159

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