Tubulin is a compound of subunits of A tubulin and B tubulin. Class III beta tubulin (beta III-tubulin) is a vertebrate tubulin isotype specific to the neurons and mammalian testis cells, making it an ideal neuronal marker. Overexpression of class III beta tubulin is associated with the resistances of microtubule-targeted cancer drugs in lung cancer cell lines, breast cancer cell lines, and ovarian tumors.