Transmission electron micrograph of cross-sectioned microtubules incubated with DAPI. The DNA-fluorochrome DAPI binds to the surface of microtubules and enables the binding of additional protofilaments (see Vater W., Böhm K.J., Unger E.: Effects of the fluorescence dye DAPI on microtubule structure in vitro: Formation of novel types of tubulin assembly products. Acta histochem. 94 (1993) 54-66).