Tubulin and paclitaxel. Molecular model of a tubulin dimer with a molecule of the chemotherapy drug paclitaxel (marketed as Taxol, ball and stick molecule at centre left) bound to it. The beta-tubulin subunit (gold) is at top and the alpha-tubulin subunit (black) is at bottom. Polymers of alpha- and beta-tubulin make up microtubules, part of the cell's cytoskeleton. Microtubules help to maintain cell shape, allow some cellular mobility and are involved in intracellular transport and nuclear division. Taxol stabilises microtubules so that they cannot disassemble and so cannot move chromosomes around the cell.