Copper chaperone of superoxide dismutase (CCS) transfers a copper(I) atom to a SOD1 monomer that already contains a Zn atom. After initial heterodimerization between SOD1 and CCS, the copper atom is transferred, intramolecular cysteine disulfide bonds are formed in SOD1, and SOD1 dimerizes (Banci et al. 2012, Casareno et al. 1998, Culotta et al. 1997, Rae et al. 2001, Brown et al. 2004, Carroll et al. 2006, Kawamata and Manfredi 2008). The transfer of copper to SOD1 requires oxygen but it is unknown at which step the oxygen acts (Brown et al. 2004). There is also a CCS-independent, oxygen-independent pathway of maturation of SOD1 (Leitch et al. 2009) whose molecular details and physiological role are not well characterized.