1NP4:
reference: The crystal structure of nitrophorin 4 at 1.5 A resolution: transport of nitric oxide by a lipocalin-based heme protein., Andersen JF, Weichsel A, Balfour CA, Champagne DE, Montfort WR, Structure 1998 Oct 15;6(10):1315-27. PMID: 9782054

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