These findings paved the way to the landmark discovery that an intronic hexanucleotide GGGGCC (G4C2) repeat expansion in the chromosome 9 open reading frame 72 (C9orf72) gene is the most common cause of both familial ALS and FTD, and cemented the clinical, genetic, and molecular link between the two diseases, giving rise to the C9orf72-associated ALS-FTD (C9ALS-FTD) disease spectrum [6,7]. Here, C9orf72 is linked to amyotrophic lateral sclerosis.