The genetic cause of amyotrophic lateral sclerosis (ALS) is known in a minority of cases. Mutations in SOD1, the gene encoding a superoxide dismutase on chromosome 21, are indeed found in 20% of familial ALS patients, who constitute only 5 or 10% of all ALS patients. In rare cases, a mutation in NFH, the gene encoding the heavy subunit of neurofilament, is present. Familial ALS has been linked to other loci but the genes involved remain to be identified. A genetic component is also thought to at least contribute to the pathogenesis of sporadic ALS. Their identification is now possible thanks to progress in molecular genetics.