C9orf72 and amyotrophic lateral sclerosis: At least 50 potential causative or disease-modifying genes have been linked to ALS5, but a G4C2 hexanucleotide repeat expansion in the C9orf72 gene is the most common ALS-causing mutation identified to date, accounting for about 40% of fALS cases and 6 to 8% of sALS cases in Caucasian populations6.