1VCC:
reference: Crystal structure of the amino-terminal fragment of vaccinia virus DNA topoisomerase I at 1.6 A resolution., Sharma A, Hanai R, Mondragon A, Structure 1994 Aug 15;2(8):767-77. PMID: 7994576

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