Before identification of the genetic lesion underlying chromosome 9-linked ALS and FTD, mutations in the SOD1 gene were the most common known genetic cause of sporadic ALS (accounting for 0·7% of cases in a population-based cohort),3 whereas mutations in the PGRN gene were the most common known cause of sporadic FTD (3·0–4·0% in clinic referral series).21 Here, GRN is linked to frontotemporal dementia.