C9orf72 and amyotrophic lateral sclerosis: Extensive studies have identified that the aberrantly expanded hexanucleotide repeat GGGGCC (HRE G4C2) located in the first non-coding region of C9orf72 gene is the most common genetic cause of ALS and FTD (5–7), and tremendous progress toward understanding disease mechanisms and developing therapies for ALS/FTD has been made (8–10).