C9orf72 and amyotrophic lateral sclerosis: Several mutations in seemingly unrelated genes, e.g., superoxide dismutase 1 (SOD1), chromosome 9 open reading frame 72 (C9orf72), or TAR DNA-binding protein 43 (TARDBP), have been described and it remains possible that ALS is not a single disease entity but a conglomerate of different diseases with a similar clinical endpoint.