It was later shown that the mutation is an expansion of GGGGCC (G4C2) hexanucleotide repeat in a noncoding region of the C9orf72 gene [14–16], which is the most common genetic cause of ALS and FTD (so called C9FTD/ALS) in certain populations. The gene discussed is C9orf72; the disease is amyotrophic lateral sclerosis.