C9orf72 and frontotemporal dementia: Additionally, the prevalence of C9orf72 HRE in this study was 143 per 100 000 (1 in 699), consistent with existing studies suggesting 1 in 517–699.1,9 Risk of ALS, FTD and any-cause dementia in C9orf72 HRE carriers was approximately doubled by the homozygosity for the C allele of rs12608932 in UNC13A.