In Amyotrophic Lateral Sclerosis (ALS) and Frontotemporal Degeneration (FTD), the presence of a hexanucleotide expansion of > 30 GGGGCC repeats (termed G4C2) within the C9orf72 gene is the most prominent mutation in familial disease [17, 65]. Here, C9orf72 is linked to frontotemporal dementia.