Higher CSF NFL levels have been reported in cases of probable or definite FTLD with TDP-43 pathology (FTLD-TDP) compared with cases of FTLD with tau pathology (FTLD-tau), whereby definite cases are pathologically proven or carry a pathology-causing mutation (GRN or C9orf72 for FTLD-TDP, MAPT for FTLD-tau) while probable cases are clinically defined (with FTD-MND and svPPA pointing to TDP-43 pathology and PSP and CBS pointing to tau pathology). Here, C9orf72 is linked to frontotemporal dementia.