C9orf72 and amyotrophic lateral sclerosis: Many RNA repeats form unusual secondary structures that have a wide range of biological or pathological effects, such as CAG and CUG hairpins in polyglutamine diseases and DM1, respectively [44,45], and GGGGCC G-quadruplex in C9orf72-ALS/FTD (amyotrophic lateral sclerosis/frontotemporal dementia) [46].