C9orf72 and amyotrophic lateral sclerosis: We now know that the most common monogenic mutated gene in Caucasians with ALS is the GGGGCC hexanucleotide repeat expansion in the C9orf72[13], [14], which is responsible for over 30% of fALS cases, and about 6.0% of sALS in Caucasian populations [7], [15], [16].