The expansion of a GGGGCC hexanucleotide repeat sequence located in a noncoding region of the C9orf72 gene was discovered several years ago (1, 2) and is the most common genetic cause of amyotrophic lateral sclerosis (ALS)2 and frontotemporal dementia (FTD) (3, 4). Here, C9orf72 is linked to frontotemporal dementia.