The MR/DM electron density maps of the O2reduction sites, obtained from the four data sets of monomer A.A–D, from data sets, IO10, IO20, IO80, and the fully reduced form (PDB code 5B1B), respectively. Structures of proteins and hemes are drawn as stick models, and the Fe atoms in heme a3 and Cu atoms in CuB site are indicated by beige and blue spheres, respectively. Resolutions of the MR/DM maps were 1.74 Å, 1.84 Å, 1.76 Å, and 1.60 Å, respectively. Electron density cages of the MR/DM maps were drawn at 1.5 σ.