In   eukaryotes there are six members of the tubulin superfamily. α- and   β-tubulins polymerize into microtubules, a major component of the eukaryotic   cytoskeleton. Microtubules function in many essential cellular processes,   including mitosis. Both α and β tubulins have a mass of around 50 kDa.   Tubulin was long thought to be specific to eukaryotes. Beta-tubulin gene is   commonly   used as a loading control for   western blot.