There are five tubulins in human cells: alpha, beta, gamma, delta, and epsilon. Tubulins are conserved across species. They form heterodimers, which multimerize to form a microtubule filament. An alpha and beta tubulin heterodimer is the basic structural unit of microtubules. The heterodimer does not come apart, once formed. The alpha and beta tubulins, which are each about 55 kDa MW, are homologous but not identical. Alpha, beta, and gamma tubulins have all been used as loading controls. Tubulin expression may vary according to resistance to antimicrobial and antimitotic drugs.