The small percentage (<1%) of HRE C9orf72 alleles described in AD patients is probably due to the phenotypic heterogeneity of C9-FTD (that involves mnemonic dysfunction) leading to misdiagnosis, and to the known incomplete penetrance (Cacace et al., 2013; Harms et al., 2013). Here, C9orf72 is linked to Alzheimer disease.