3NR6:
reference: Crystal structure of XMRV protease differs from the structures of other retropepsins., Li M, Dimaio F, Zhou D, Gustchina A, Lubkowski J, Dauter Z, Baker D, Wlodawer A, Nat Struct Mol Biol. 2011 Jan 23. PMID: 21258323

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