Copper chaperone of superoxide dismutase (CCS) transfers a copper(I) atom to a SOD1 monomer that already contains a Zn atom (Culotta et al. 1997, Casareno et al. 1998, Rae et al. 2001, Brown et al. 2004, Banci et al. 2012). The reaction proceeds by a two step mechanism in which SOD1 first forms heterodimers with CCS (Rae et al. 2001, Banci et al. 2012).