C9orf72 and amyotrophic lateral sclerosis: The intronic hexanucleotide repeat expansion (GGGGCC) in the C9orf72 gene is the most common genetic cause of both ALS and FTD, affecting ~34% of fALS and ~5% of sALS cases in Europeans and ~2% of fALS and <1% of sALS cases in Asians [90].