Cytochrome c oxidase (CcO) catalyzes the four-electron reduction of molecular oxygen to water and couples it to the transmembrane proton transfer in the respiratory chain of mitochondria and bacteria. The CuA site functions as the proximate electron-transfer intermediate from an electron source in CcO. It has characteristic structural features, providing an electronic structure suitable for rapid electron transfer. X-ray crystallographic structures show the structural variety of the Cu2S2 core of the CuA site. On page 3756, Yu Takano and colleagues address the origin of the structural variety of the Cu2S2 core of the CuA site, using the density functional theory.