C9orf72 and amyotrophic lateral sclerosis: ALS is caused by mutations in more than 50 disease-modifying genes, most prevalent ones include mutations in copper/zinc superoxide dismutase type-1 (SOD1), chromosome 9 open reading frame 72 (C9orf72), TAR DNA-binding, and fused in sarcoma (FUS), which account for 25% of familial ALS cases [2].