About 5 to 10 percent of ALS is familial — meaning it arises in families in which there is a history of ALS. Several genes associated with ALS have been identified or at least mapped to a specific region of a chromosome.
The other 90 to 95 percent of ALS is sporadic, meaning it occurs without a family history (in other words, "sporadically"). There appear to be genetic variations that influence one's susceptibility to sporadic ALS, even if they don't actually cause the disease by themselves.