1NP1:
reference: Crystal structures of a nitric oxide transport protein from a blood-sucking insect., Weichsel A, Andersen JF, Champagne DE, Walker FA, Montfort WR, Nat Struct Biol 1998 Apr;5(4):304-9. PMID: 9546222

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