The human copper metallochaperone (CCS) for Cu,Zn-superoxide dismutase (SOD1) has a similar Zn(II) site as that in SOD1. Dimeric CCS converts to a monomer in the reduced Zn(II)-free form that weakens the interaction with SOD1. This form of CCS may be fibrillogenic and disease causing, as is the case for demetallated and reduced monomeric SOD1.