Our results reinforce previous works suggesting that the size of polyQ associated STRs influences disease outcomes in TDP-43 associated diseases.10,11,19,32 More recently, IA-HTTs have been shown to influence the course of the disease in non-genetic forms of ALS.33 Our results suggest that IA-HTT are also modifiers of C9orf72 FTD/ALS. Here, C9orf72 is linked to amyotrophic lateral sclerosis.