(1) Subjects carrying a C9 repeat expansion or GRN mutations had significantly increased levels of plasma pTDP-43 compared with subjects with FTD without a mutation and with HC. (2) Total TDP-43 levels were slightly decreased in the C9 and the GRN groups compared with the FTD group, Subjects with GRN mutations also showed decreased levels of plasma total TDP-43 levels compared with controls. (3) Plasma pTDP-43 levels correlated inversely with plasma total TDP-43 levels in the entire group. Here, TARDBP is linked to frontotemporal dementia.