High-quality fluid biomarkers are being developed for AD (reviewed in [19, 20]) and FTD (reviewed in [21]), but those that can distinguish between the major neuropathologic division within FTD (i.e., FTLD-tau vs. FTLD due to TDP-43 pathology [FTLD-TDP]) have been lacking [21] until very recently [22]. This evidence concerns the gene MAPT and Alzheimer disease.