A GGGGCC hexanucleotide repeat expansion (HRE) in the first intron of the chromosome 9 open reading frame 72 (C9orf72) gene was discovered to be the most frequent cause of both frontotemporal dementia (FTD) and amyotrophic lateral sclerosis (ALS), as well as mixed FTD/ALS phenotype. The gene discussed is C9orf72; the disease is amyotrophic lateral sclerosis.