C9orf72 and amyotrophic lateral sclerosis: The most common genetic mutation underlying ALS and FTD is the C9orf72 hexanucleotide repeat expansion (HRE), which pathologically presents as G4C2 repeat RNA foci and sense and anti-sense repeat-associated non-ATG (RAN) translation of di-peptide repeat proteins (DPRs) as well as TDP-43 aggregation.