In SOD1‐linked familial ALS patients, mutant SOD1 is expressed ubiquitously throughout SOD1‐linked familial ALS patients, yet mutant SOD1 misfolding and toxicity manifests selectively within upper and lower motor neurons in these patients, and very rarely occurs before mid‐way through their fourth decade of life.[211] In Parkinson's disease patients the accumulation of misfolded SOD1 protein in the vulnerable SNc occurs in the absence of known SOD1 gene mutations.[166a] Together these data suggest that non‐genetic factors are important contributors to the disease process in both disorders. Here, SOD1 is linked to amyotrophic lateral sclerosis.