We recently reported that a large hexanucleotide repeat expansion located within the non-coding portion of C9orf72 is the cause of chromosome 9-linked ALS and FTD.9, 10 This genetic lesion accounted for a large proportion (∼40·0%) of familial cases of ALS and FTD. Here, C9orf72 is linked to amyotrophic lateral sclerosis.