Tubulin is a major cytoskeleton component that has five distinct forms, designated ?, ?, ?, ? and ? Tubulin. ? and ? Tubulins form heterodimers which multimerize to form a microtubule filament. Multiple ? Tubulin isoforms (?1, ?2, ?3, ?4, ?5, ?6 and ?8) have been characterized and are expressed in mammalian tissues. ?1 and ?4 are present throughout the cytosol, ?2 is present in the nuclei and nucleoplasm, and ?3 is a neuron-specific cytoskeletal protein. ? Tubulin forms the gammasome, which is required for nucleating microtubule filaments at the centrosome. Both ? Tubulin and ? Tubulin are associated with the centrosome. ? Tubulin is a homolog of the Chlamydomonas ? Tubulin Uni3 and is found in association with the centrioles, whereas ? Tubulin localizes to the pericentriolar material. ? Tubulin exhibits a cellcycle-specific pattern of localization, first associating with only the older of the centrosomes in a newly duplicated pair and later associating with both centrosomes.