1NIH:
reference: Structure of deoxy-quaternary haemoglobin with liganded beta subunits., Luisi B, Liddington B, Fermi G, Shibayama N, J Mol Biol 1990 Jul 5;214(1):7-14. PMID: 2370669

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