Tau schematic. The longest isoform of tau, 2N4R, is shown. Regions of interest indicated are the N-terminal domain, the proline rich region (PRR), the microtubule binding region (MTBR), and the C-terminal domain. The residues mutated to cysteine for labeling span these regions of interest and are indicated in the schematic. Alternative splicing of N1, N2, and R2 (marked with dashed lines) result in the six isoforms of tau found in human adults.