ALS with frontotemporal dementia (ALS-FTD) is often caused by a polymorphic hexanucleotide repeat expansion (HRE) of GGGGCC (G4C2) in the first intron of the C9orf72 gene from normally less than 30 repeats into the hundreds or thousands, pathologically. Here, C9orf72 is linked to amyotrophic lateral sclerosis.