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It has long been debated whether the multiple tubulins found in most organisms subserve different functions (Fulton and Simpson, [1976]. In: Cell Motility, eds. Goldman, R., Polland, T., and Rosenbaum, J, New York: Cold Spring Harbor Press, 987-1006). The debate was further fueled by the discovery that β-tubulin from higher organisms consists of 7 distinct isotypes (differing most notably in their last 15 amino acids), that these carboxyl terminal sequences are highly conserved across all vertebrate species, and that there is differential production of these isotypes in various tissues (Sullivan [1988]. Ann. Rev. Cell Biol. 4, 687-716). Despite these observations, transfection of specific β-tubulin cDNAs into mammalian cells has demonstrated that most β-tubulins are used interchangeably in microtubule assembly and produce no physious phenotype in the transfected cells (Joshi and Cleveland [1990]. Cell Motil. Cytoskel. 16, 159-163; Blade et al., [1999]. J. Cell Sci. 112, 2213-2221). Contrary to these earlier studies, an article by Bhattacharya and Cabral (Mol. Biol. Cell [2004] 15, 3123-3131) now reports that transfection and overproduction of a minor, ubiquitously expressed, class V β-tubulin results in a dramatic phenotype. The cover shows immunofluorescence of transfected Chinese hamster ovary cells. Compared to normal cells (green), the transfected cells (red) have lost most of their microtubules and contain many microtubule fragments. As a result of this disrupted microtubule assembly, defective mitotic spindles are formed, cell division is inhibited, and the cells exit mitosis to become progressively larger, flatter, and multinucleated (blue) with each subsequent cell cycle. Because class V β-tubulin is a minor isotype found at almost all times, the authors speculate that it may play a basic role, such as regulating the dynamics or remodeling of the microtubule cytoskeleton. —Fernando Cabral