SOD1 and amyotrophic lateral sclerosis: Among patietns with SOD1 mutations, the classic ALS phenotype was observed in 56% of patients (both upper and lower motor neuron involvement; UMN, LMN), 41% showed progressive muscle atrophy (PMA, isolated signs of LMN), and 3% (n = 1) a mixed ALS-MSA-P (multiple system atrophy-parkinsonism) phenotype.