C9orf72 and frontotemporal dementia: In several studies that directly compare persons with C9orf72 expansions to those without C9orf72 expansions, a higher frequency of subsequent feature development has been reported in individuals with C9orf72 expansions (23–50%) compared to those without C9orf72 expansions (4–12%).26-28 However, research limited to individuals with C9orf72 expansions likely overestimates concomitant FTD and ALS.