MAPT and Alzheimer disease: The insoluble tau 4R/3R ratio classification (Fig. 3a) separated subjects in three groups, namely 4R-exclusive (all five CBD and four FTLD cases, including two with the P301L MAPT mutation (subjects 35 and 37)), 3R-exclusive (all five PiD and six FTLD cases) or both 4R- and 3R-tau isoform aggregates (AD and CTL cases), highlighting heterogeneity amongst the FTLD cases and justifying the subdivision into FTLD-4R and FTLD-3R groups across all subsequent analyses.