C9orf72 and amyotrophic lateral sclerosis: The most studied ALS-linked genes are chromosome 9 open reading frame (C9orf72), Cu2+/Zn2+ superoxide dismutase 1 (SOD1), TAR DNA binding protein (TARDBP), and fused in sarcoma (FUS), which collectively account for nearly 48% and 5% of familial and sporadic cases, respectively [23].