C9orf72 and frontotemporal dementia: In addition, G-quadruplexes have important roles in neurological diseases, including G-quadruplex-forming repeat expansions in the C9orf72 gene in frontotemporal dementia and amyotrophic lateral sclerosis [35, 36] and loss of the G-quadruplex binding protein FMRP in the intellectual disability fragile X syndrome [37].