C9orf72 and frontotemporal dementia: In biological systems, for example, aberrant amplification of the hexanucleotide GGGGCC (G4C2) repeated in the human C9ORF72 gene is the most common genetic factor found behind frontotemporal dementia (FTD) and amyotrophic lateral sclerosis (ALS) (DeJesus-Hernandez et al., 2011).