C9orf72 and amyotrophic lateral sclerosis: Age-dependent neurodegenerative disease ALS is caused by the degeneration of motor neurons, commonly associated to protein inclusions resulting from pathogenic variants in four main genes: superoxide dismutase 1 (SOD1), TAR DNA-binding protein 43 (TDP-43), RNA-binding protein FUS, and C9orf72 [84,181] (Table 1).