We report a new missense mutation (Ala140Gly) in exon 5 of the Cu/Zn superoxide dismutase (SOD-1) gene in a 73-year-old man with familial amyotrophic lateral sclerosis (FALS). The enzymatic activity of mutated SOD-1 measured in erythrocyte lysate was 70% of control. This heterozygote mutation, which is associated with the late onset of the disease, is located in the active site of the enzyme.