C9orf72 and amyotrophic lateral sclerosis: The pathological hexanucleotide expansion GGGGCC located in the first intron or promoter region of the C9orf72 gene on chromosome 9p21 represents the most common genetic cause of amyotrophic lateral sclerosis (ALS) and frontotemporal lobar dementia (FTLD)—two fatal neurodegenerative diseases with progressive loss of motor neurons in the brain and spinal cord [1,2,3,4,5,6,7].