ATXN2 and amyotrophic lateral sclerosis: For example, the intermediate length trinucleotide CAG repeats encoding polyglutamine tracts in the ATXN2 gene that confer a risk of developing ALS have been associated with C9orf72 repeat expansion carriers with ALS but not with those carriers with FTLD.9 10 Environmental factors have also been implicated in ALS pathogenesis, but it has proven difficult to replicate these experimental outcomes reliably.