The genetic inquiry in the diagnostic process of amyotrophic lateral sclerosis (ALS) need to be precise and systematic. Several hereditary neurologic disorders may initially mimic ALS, such as dominant spino-cerebellar ataxia or spastic paraplegia. Other hereditary motor neuron disorders are clinically more difficult to distinct from ALS such as Kennedy's disease, adult-onset spinal muscular atrophies or juvenile ALS. When a final diagnostic of ALS is established, the genetic inquiry aims at identifying a familial ALS (FALS) case. Almost 10 to 20 percent of ALS cases are familial. It is now admitted that 10 to 20 percent of those cases are due to SOD1 gene mutations. Such mutations may have a dominant or recessive inheritance. They are mainly found in multigenerational families. In 80 percent of the FALS cases, only two ALS cases are found in the pedigree. One cannot know whether this represent dominant cases with low penetrance, recessive inheritance or a multigenic, hereditary complex, disorder. More other, not all the SOD1 mutations have been demonstrated as causal. These elements are strong enough to suggest to precisely study, in the presence of a given SOD1 mutation, both the clinical phenotype, the data from the literature and, as often as possible, the segregation of the mutation into the family, before ascertaining that the mutation is responsible for a hereditary case of ALS. In every case, the genetic inquiry together with its conclusions should be done with caution, taking into account both the patient's need of a clear information and his anxiety regarding his descent.