The transition from interphase to mitosis is marked by a dramatic change in microtubule dynamics resulting in the reorganization of the microtubule network that culminates in mitotic spindle formation. While the molecular basis for this change in microtubule organization remains obscure, it is currently thought that a balance in the activity of microtubule stabilizing and destabilizing factors regulates how dynamic cellular microtubules are. By mixing the microtubule stabilizer XMAP215 and the microtubule destabilizer XKCM1, reconstitution of in vivo-like microtubule dynamics has now been achieved in vitro.(1) BioEssays 24:305–307, 2002. ©2002 Wiley Periodicals, Inc.