3PKM:
reference: Interaction of the Cas6 Riboendonuclease with CRISPR RNAs: Recognition and Cleavage., Wang R, Preamplume G, Terns MP, Terns RM, Li H, Structure. 2011 Feb 9;19(2):257-64. PMID: 21300293

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