Longitudinal changes in plasma NfL in presymptomatic and symptomatic GRN and C9orf72 mutation carriers and controls (aged 21–83 years) were measured in cohorts from the French Research Network on FTD/FTD-ALS (Inserm RBM02–59), Predict to Prevent Frontotemporal Lobar Degeneration and ALS (PREV-DEMALS), and Predict-PGRN.5 Subjects were followed for more than 2 years, and the mean annualized rate of change (ARC) of plasma NfL was calculated along with absolute levels of plasma NfL at baseline and follow-up. This evidence concerns the gene NEFL and amyotrophic lateral sclerosis.