C9orf72 and frontotemporal dementia: A hexanucleotide GGGGCC (G4C2) repeat expansion in the C9orf72 gene is the most common cause of amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD) (DeJesus-Hernandez et al., 2011; Renton et al., 2011).