MAPT and tauopathy: One ofthe four repeated sequences is encoded by exon 10; inclusion of this sequence leadsto 4 repeat (4R) tau isoforms, while exclusion leads to 3 repeat (3R) isoforms.Similar amounts of 3R and 4R tau are present in normal human brain, whereaspathological tau may be predominantly composed of 4R, 3R or both isoforms.20,27 Depending on the predominant tau isoform found infilaments, FTLD-tau subtypes are classified as 3R, 4R or 3R/4R tauopathies (Table 1).