The initial screening of expansion of G4C2 repeats near ALS loci found that though such repeats are quite common in human genome, the expansion of G4C2 repeats were only detectable in C9ORF72 gene but not in other ALS loci genes [27], indicating that the context of the expanded repeats at C9ORF72 gene are quite unique for expanded repeat toxicity. Here, C9orf72 is linked to amyotrophic lateral sclerosis.