The tubulin family of globular proteins has several members, the most common of which are a-tubulin and ?tubulin; proteins which make up microtubules of the cytoskeltons of probably all eukaryotic cells. Except in the simplest eukaryotes, tubulin (100 kDa) exists in all cells as a heterodimer of two similar but non-identical polypeptides (55 kDa each), designated alpha and beta. Within either family of alpha/beta tubulin heterodimers, individual subunits diverge from each other (both within and across species) at less than 10% of the amino acid positions. The most extreme diversity is localized to the carboxyl-terminal 15 residues. Delta (d) and epsilon (e) tubulin have been found to localize at centrioles and may play a role in forming the mitotic spindle during mitosis, though neither is as well-studied as the a- and ? forms.