C9orf72 and frontotemporal dementia: A hexanucleotide (GGGGCC) expansion in the first intron of the C9orf72 gene accounts for approximately 40% of cases of familial amyotrophic lateral sclerosis (ALS), up to 7% of sporadic ALS, and approximately 20% of familial frontotemporal lobar degeneration, establishing a firm genetic link between ALS and frontotemporal dementia (FTD) 1, 2, 3.