C9orf72 and amyotrophic lateral sclerosis: Pathologically, ALS-associated genetic signatures vary in frequency, but the most common mutations occur in four genes: chromosome 9 open reading frame 72 (C9ORF72), TAR DNA-binding protein (TARDBP), superoxide dismutase 1 (SOD1), and fused in sarcoma/translocated in liposarcoma (FUS).