A search for genes involved in ALS/FTD led to the discovery of a hexanucleotide-repeat expansion mutation in the first intron of C9ORF72. In a North America cohort, this mutation underlies 11.7% and 23.5% of familial FTD and ALS cases, respectively, and in a large Finnish population, the C9ORF72 mutation underlies 46.0% of familial ALS and 21.1% of sporadic ALS (DeJesus-Hernandez et al., 2011; Renton et al., 2011). Here, C9orf72 is linked to amyotrophic lateral sclerosis.