MAPT and the six τ isoforms expressed in adult human brain. MAPT consists of 14 exons (E). Alternative mRNA splicing of E2 (red), E3 (green) and E10 (yellow) gives rise to six τ isoforms (352–441 amino acids). Constitutively spliced exons (E1, E4, E5, E7, E9, E11, E12, E13) are shown in dark blue. E6 and E8 (violet) are not transcribed in human brain. E4a (orange) is only expressed in the peripheral nervous system. The repeats (R1–R4) are shown, with three isoforms having four repeats (4R) and three isoforms having three repeats (3R).