In particular, our interest is focused on plasma biomarkers to help distinguish between MCI + AD and controls and between FTD and progressive supranuclear palsy (PSP); i.e., P-tau181 [122]—the positive correlation between CSF and plasma values for NfL (p < 0.0001), with NfL values higher for all phenotypes of symptomatic FTD-ALS spectrum (FAS) patients compared to primary psychiatric disorders (PPD) [121]. This evidence concerns the gene NEFL and Classical progressive supranuclear palsy.