SOD1 and amyotrophic lateral sclerosis: We found that extracellular misfolded SOD1 exists as a metal-free and disulfide oxidized form (apo-SOD1S-S) in the conditioned medium from NSC-34 cells expressing hSOD1WT or the ALS-causing hSOD1 mutants (G93A, G37R, D90A, and G85R) (Figure 5a,b,e).