Anti-microtubule agents are plant-derived chemicals that block cell division by preventing microtubule function. Microtubules are an important cellular structure composed of two proteins, α-tubulin and β-tubulin. They are hollow, rod-shaped structures that are required for cell division, among other cellular functions. Microtubules are dynamic structures, which means that they are permanently in a state of assembly and disassembly. "Vinca" alkaloids and taxanes are the two main groups of anti-microtubule agents, and although both of these groups of drugs cause microtubule dysfunction, their mechanisms of action are completely opposite: "Vinca" alkaloids prevent the assembly of microtubules, whereas taxanes prevent their disassembly. By doing so, they prevent cancer cells from completing mitosis. Following this, cell cycle arrest occurs, which induces programmed cell death (apoptosis). These drugs can also affect blood vessel growth, an essential process that tumours utilise in order to grow and metastasise.