It has been demonstrated that C9orf72 HRE G4C2 forms G-quadruplex structures in vitro and the structural polymorphism of C9orf72 HRE G4C2, including G-quadruplex structures at both DNA and RNA level, is implicated in the development of ALS and FTD (56). Here, C9orf72 is linked to amyotrophic lateral sclerosis.