It is also possible that the “wheel-like” microtubule networks we often observed con- founded our assignment of motor direction as compared to those of the “aster-like” networks of non-transfected cells. The expres- sion of tau may also promote spontaneous microtubule nucleation without centrosomes, which would disrupt the normal polarity of microtubules found in control cells (Witman et al., 1976).