C9orf72 and frontotemporal dementia: Hexanucleotide repeat expansions (GGGGCC) in a non-coding region of C9orf72 are the most common genetic abnormality in both ALS and FTD, which is responsible for approximately 40% of familial ALS, 5–10% of sporadic ALS, 40% of familial FTD, and 4–21% of sporadic FTD [13,14,15].