SOD1 (Superoxide dismutase 1) destroys free radicals which are normally produced within the cells and which are toxic to biological systems. SOD1 catalyzes the following reaction: 2 superoxide + 2 H+ = O2 + H2O2. Defects in SOD1 are the cause of amyotrophic lateral sclerosis type 1 (ALS1). ALS1 is a familial form of amyotrophic lateral sclerosis, a neurodegenerative disorder affecting upper and lower motor neurons and resulting in fatal paralysis. Sensory abnormalities are absent. Death usually occurs within 2 to 5 years. The etiology of amyotrophic lateral sclerosis is likely to be multifactorial, involving both genetic and environmental factors. The disease is inherited in 5-10% of cases leading to familial forms.