About 10 p. cent of amyotrophic lateral sclerosis (ALS) cases are familial. Most of the familial ALS (FALS) cases are clinically homogeneous. Among these families, autosomal dominant, X-linked or autosomal recessive transmission can be observed. Most of the causal mutations have been observed in the SOD1 gene. To date, more than one hundred different mutations have been described, but it remains unclear whether the mutation is always responsible for the phenotype. Penetrance of the mutation depends on age, with almost 90 p. 100 of penetrance at age 70 years. There is no anticipation. Worldwide, the most frequent mutation is A4V with dominant transmission, responsible for a severe, rapid form of the disease. The second most frequent mutation is D90A which is generally transmitted recessively, predominantly in the Scandinavian countries. The phenotype is characterized by a long lasting course (mean: 11 years). Other causal mutations have been described in the Alsine, Apex, NF-H and NAIP genes. Other genes can be considered as risk factors, like SMN2, APO E4, APEX, Dynactine, P-450 D6. Presymptomatic testing for FALS seems difficult because little information can be given to the patient regarding the responsibility of the mutation in the disease, age of onset, and disease trends. The same precautions as for Huntington's disease are needed. Genetic investigations can contribute to better understanding of the pathophysiology of ALS. Other causal genes in the 90 p. 100 of FALS without SOD1 mutation and eventually in the sporadic ALS cases may be disclosed. Genetic investigations also determine the precise role of a given SOD1 mutation because of the large number of potential SOD1 mutations, the variability of the transmission mode and the non-exceptional absence of proven causality for ALS.