C9orf72 and amyotrophic lateral sclerosis: Much of this controversy may have been resolved as some ALS risk genes such as C9orf72 causing G4C2 repeat expansion, are involved in the response to exercise, and exercise with this genotype shortens the age of onset, so exercise-induced ALS is associated with a risk-genotype [286], with the possibility that a certain dose of exercise may induce ALS in subjects with C9orf72 expansions.