2GMI:
reference: Mms2-Ubc13 covalently bound to ubiquitin reveals the structural basis of linkage-specific polyubiquitin chain formation., Eddins MJ, Carlile CM, Gomez KM, Pickart CM, Wolberger C, Nat Struct Mol Biol. 2006 Oct;13(10):915-20. Epub 2006 Sep 17. PMID: 16980971

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