Tau phosphomutants. The diagram shows the positions in the longest human tau isoform (2N4R, 441 residues) of the serine (S) or threonine (T) residues mutated to glutamate (to mimic phosphorylation) in E18tau and E27tau, or to alanine (to preclude phosphorylation) in A18tau. R1, R2, R3, and R4 are the repeat regions of the microtubule (MT)-binding domain.