Microtubules are hollow fibres that form the track upon which chromosomes or proteins, such as kinesins, are transported in the cell. They are formed by the self-assembly of the protein tubulin both in vitro and in vivo. In the cell, their appearance in time and space is strictly controlled by the presence of nucleation centres. Microtubules are very dynamic structures, a property that is obtained by coupling the self-assembly process to the hydrolysis of the nucleotide, guanosine 5'-triphosphate, (GTP). After assembly, GTP is hydrolysed and guanosine 5'-diphosphate, (GDP)-microtubule structure is formed which, although intrinsically very unstable, is stabilised by a small remaining tubulin-GTP-cap at both ends. As such, the ends of microtubules can be considered as gates for entry into the polymeric state. These gates can be blocked by sub-stoichiometric amounts of drugs such as colchicine.