C9orf72 and frontotemporal dementia: An intronic GGGGCC hexanucleotide repeat expansion in the chromosome 9 open reading frame 72 (C9orf72) gene is the most common monogenic form of amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD), accounting for approximately 25–40% of familial cases (1–3).