C9orf72 and amyotrophic lateral sclerosis: Familial ALS (fALS) cases account for approximately 10% of all ALS cases; approximately 40% of fALS cases and a proportion of sporadic cases are caused by the C9orf72 hexanucleotide G4C2 repeat expansion, making it the commonest known ALS genetic mutation (DeJesus-Hernandez et al., 2011).