Tubulins, major components of the cellular cytoskeleton, form microtubules that are critical for cellular structure, migration, cell division and intracellular trafficking. There are at least three types of tubulins, α, β and γ. Microtubules are large structures (300 angstrom diameter) composed of alternating α and β subunits. Compounds that block microtubule formation, such as Taxol and the vinca alkaloids, inhibit cell division and are effective anti-cancer agents.