Sporadic - the most common form of ALS in the United States - 90 to 95% of all cases.
Familial - occurring more than once in a family lineage (genetic dominant inheritance) accounts for a very small number of cases in the United States - 5 to 10% of all cases.
Guamanian - an extremely high incidence of ALS was observed in Guam and the Trust Territories of the Pacific in the 1950's.
The most common form of ALS in the United States is "sporadic" ALS. It may affect anyone, anywhere. "Familial" ALS (FALS) means the disease is inherited. Only about 5 to 10% of all ALS patients appear to have genetic or inherited form of ALS. In those families, there is a 50% chance each offspring will inherit the gene mutation and may develop the disease.