C9orf72 and amyotrophic lateral sclerosis: An important landmark study in understanding the genetic basis of ALS is the discovery of the GGGGCC hexanucleotide repeat expansion in the C9orf72 gene, which appears to be the most common genetic cause of the disease in Europe and North America [30,31], representing 45–50% of fALS and 5–10% of sALS cases [32].