Twenty percent of cases of familial amyotrophic lateral sclerosis (FALS) have identifiable mutations in the gene for Cu, Zn superoxide dismutase (SOD1) located on the long arm of chromosome 21. SOD1 mutations are thought to cause a yet unknown toxic gain of function resulting in motor neuron damage. Seventy-one mutations, located in all five exons of SOD1, have been reported. Identified mutations are predominantly heterozygous mis-sense mutations, although rare nonsense mutations, deletions, and insertions exist. While gene dosage has an effect on the age of onset, genotoype/phenotype correlation is better defined for progression of symptoms than for disease onset.