It is true that up to 10% of patients have a genetic predisposition to the disease. The majority of patients have what is called sporadic ALS. We do not know what triggers ALS in these patients. Genes have been identified in what we call familial ALS, that is in cases where ALS is inherited. Some of these genes are also present in patients with sporadic ALS. Although rare, the same mutations are found in sporadic cases. The cause is unknown in most sporadic cases. According to my understanding of the illness, there is an interplay of the factors of genetic predisposition, aging and environmental aspects. The interplay of these three factors triggers the illness at a given point in the life of an individual.
On November 2nd, 2010. See this statement in context.