C9orf72 and amyotrophic lateral sclerosis: The non-coding hexanucleotide GGGGCC repeat expansion mutation in the chromosome 9 open reading frame 72 (C9orf72) gene is a major genetic cause of amyotrophic lateral sclerosis (ALS), a progressive and fatal neurodegenerative disorder characterised by a loss of motor neurons and paralysis1,2.