3GPM:
reference: A charged residue at the subunit interface of PCNA promotes trimer formation by destabilizing alternate subunit interactions., Freudenthal BD, Gakhar L, Ramaswamy S, Washington MT, Acta Crystallogr D Biol Crystallogr. 2009 Jun;65(Pt 6):560-6. Epub 2009, May 15. PMID: 19465770

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