The microtubules of an axon are thought to be nucleated at the centrosome, then severed from their nucleation site and moved into the axon. It was noted in the text that cytoplasmic dynein is responsible for the retrograde movement of organelles in axons, yet this same motor is thought to mediate the anterograde movement of microtubules in these same cellular processes. How is it possible that the same minus end-directed motor can be involved in both anterograde and retrograde movements?