C9orf72 and amyotrophic lateral sclerosis: In a normal person, intron one of C9orf72 contains, at most, 20–30 hexanucleotide repeats (GGGGCC; G4C2), but, in people with C9orf72-associated ALS (C9-ALS), this number can expand to hundreds to several thousand repeats, which is called hexanucleotide repeat expansion (HRE) [8,9].