Fewer than 5% of ALS patients have evidence of an inherited (familial) form of ALS (FALS). At the same time, in over 95% of cases, the risk of ALS is no higher for family members than for the normal population. The essential hallmark of whether FALS is present is the medical history of other family members. Whether the patient’s father or mother or the siblings of the parents and patient had or have ALS is of crucial importance. If there is no evidence of ALS in the family history, it can be assumed that the ALS is of a nongenetic form. Genetic testing for an inherited risk is not available for the great majority of patients. Genetic testing for a so called ALS gene with great limitations is possible in principle only in the case of FALS.

About 10% of patients with FALS, i.e. fewer than 0.5% of all ALS patients, have genetic changes (mutations) in the superoxide dismutase (SOD1) gene. In these patients, SOD1 mutations can be regarded as the cause of the ALS and are accessible for genetic testing. If FALS is present in a family, it is possible to perform a molecular genetic test for SOD1. However, this test has two important limitations:

1. The SOD1 mutations are responsible for only a small percentage of FALS (10%). The cause of the predominating 90% of FALS cases is still unknown. A negative SOD1 test when there is other evidence of FALS therefore yields no important information and consequently no diagnostic certainty.
2. Because of the low therapeutic options, the value of genetic investigation is limited. For instance, there is no long term experience for therapy with riluzole over decades. Accordingly, treatment of symptom free genetic carriers with SOD1 mutations cannot be recommended.