A large GGGGCC hexanucleotide repeat expansion in the first intron or promoter region of the C9orf72 gene is the most common genetic cause of familial and sporadic Amyotrophic lateral sclerosis (ALS), a devastating degenerative disease of motor neurons, and of Frontotemporal Dementia (FTD), the second most common form of presenile dementia after Alzheimer’s disease. This evidence concerns the gene C9orf72 and amyotrophic lateral sclerosis.