A hexanucleotide repeat expansion consisting of hundreds to thousands of G4C2 repeats located in a non-coding intronic region of chromosome 9 open reading frame 72 (C9orf72) is the most frequent genetic cause of frontotemporal dementia (FTD) and amyotrophic lateral sclerosis (ALS), collectively referred to as c9FTD/ALS. This evidence concerns the gene C9orf72 and amyotrophic lateral sclerosis.