C9orf72 and amyotrophic lateral sclerosis: In the present study, we aimed at expanding our C9orf72 observations in the Flanders-Belgian FTLD and FTLD–ALS cohort (n = 360) with a larger European cohort of 845 FTLD and FTLD–ALS patients, in which we determined the geographical distribution and prevalence of the pathological G4C2 expansion.