C9orf72 and amyotrophic lateral sclerosis: Nevertheless, the most common hereditary cause of ALS/FTD is expansion of the intronic hexanucleotide repeat (GGGGCC)n in C9orf72, accounting for about 40% of familial ALS cases (where the patient has a family history of ALS) and 7% of the sporadic cases (no family history of ALS) in Europe7.