Herpesviral VP22 proteins (and variants) are used to modify cell structure and cell division, by their newly found property of binding to microtubules in cells. Uses of VP22 to exploit this property include stabilisation of animal cellular microtubules in vivo and in vitro, e.g. to retard or arrest cell division or induce cell death. The microtubule binding function of VP22 can be exploited by reagent use in vitro to study microtubules or the cell cycle particularly at cell division, and pharmaceutically to retard or arrest cell division of cells such as neoplastic cells or protozoal parasite cells in vitro or in vivo.