MAPT and tauopathy: sEV 3R/4R tau ratios were lower in the 4R tauopathy PSP (median 0.18, IQR [0.13–0.29]; P < 0.0001 for all comparisons), and higher in bvFTD compared with all other groups (bvFTD median 2.59, IQR [2.02–3.87], P < 0.001 versus HC; P < 0.0001 versus all other groups).