A recently discovered hexamer (GGCCCC) repeat expansion in the C9orf72 locus is a major cause of both ALS and frontotemporal dementia (DeJesus-Hernandez et al. 2011; Renton et al. 2011; Gijselinck et al. 2012). Here, C9orf72 is linked to amyotrophic lateral sclerosis.