When this result was broken down by phenotype, it was striking that intermediate-length ATXN2 expansions were present in 2–3 % of patients with ALS or ALS/FTD but were absent in 75 patients with FTD, suggesting that expansion of ATXN2 may predispose C9orf72 expansion carriers to develop ALS or ALS/FTD rather than pure FTD. This evidence concerns the gene ATXN2 and amyotrophic lateral sclerosis.