C9orf72 and frontotemporal dementia: The most common known mutation associated with ALS/frontotemporal dementia (FTD) is a G4C2 hexanucleotide DNA repeat expansion in the non-coding region of C9ORF72, accounting for at least 10% of all ALS/FTD (up to 40% of familial cases, at least 10% of sporadic cases).