C9orf72 and amyotrophic lateral sclerosis: An increase in the number of d(G4C2)•(G2C4) nucleotide repeats (repeat expansion) in the non-coding region of C9orf72 gene has been identified as the most common genetic cause of two devastating neurological disorders, amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD) (1,2).