C9orf72 and amyotrophic lateral sclerosis: From the discovery of the first ALS-associated gene SOD1, several candidate-gene or genome-wide association studies (GWAS) have identified multiple single-nucleotide polymorphisms (SNPs) affecting potentially ALS-associated genes, including C9orf72, TDP43, FUS, MATR3, UBQLN2, VCP and OPTN6–9.