C9orf72 and amyotrophic lateral sclerosis: Approximately 5–10% of ALS cases are familial (familial ALS, fALS), and more than 20 causative genes have been identified from fALS, such as superoxide dismutase 1 (SOD1), TAR DNA-binding protein 43 kDa (TDP-43), fused in sarcoma (FUS), optineurin and chromosome 9 open reading frame 72 (C9orf72)3–16.