A study examining the incidence of C9orf72 mutations in amyotrophic lateral sclerosis and FTD found that the pathogenic expansion of C9orf72 hexanucleotides was non-penetrant in individuals younger than 35 years, 50% penetrant by 58 years and almost fully penetrant by 80 years.22 Furthermore, incomplete penetrance and anticipation effects must be considered. Here, C9orf72 is linked to amyotrophic lateral sclerosis.