The tubulin superfamily includes five distinct families, the alpha-, beta-, gamma-, delta-, and epsilon-tubulins and a sixth family (zeta-tubulin) which is present only in kinetoplastid protozoa. The alpha- and beta-tubulins are the major components of microtubules, while gamma-tubulin plays a major role in the nucleation of microtubule assembly. The delta- and epsilon-tubulins are widespread but unlike the alpha, beta, and gamma-tubulins they are not ubiquitous among eukaryotes. The alpha/beta-tubulin heterodimer is the structural subunit of microtubules. The alpha- and beta-tubulins share 40% amino-acid sequence identity, exist in several isotype forms, and undergo a variety of posttranslational modifications. The structures of alpha- and beta-tubulin are basically identical: each monomer is formed by a core of two beta-sheets surrounded by alpha-helices. The monomer structure is very compact, but can be divided into three regions based on function: the amino-terminal nucleotide-binding region, an intermediate taxol-binding region and the carboxy-terminal region which probably constitutes the binding surface for motor proteins.