C9orf72 and amyotrophic lateral sclerosis: However, more than 35 different genes have been linked to ALS, with the rate of gene discovery showing a quadrennial twofold increase.2 Large-scale genome-wide association studies have identified four main genes as highly frequent ALS causative factors: the superoxide dismutase (SOD1) gene, TAR DNA binding protein (TARDBP) gene, chromosome 9 open reading frame 72 (C9orf72) gene and fused in sarcoma (FUS) (Figure 3).