The protein tubulin is found in every cell of every living organism (with the exception of bacteria ) [1]. In the presence of guanosine-tri-phosphate (GTP) and magnesium ions (Mg +), tubulin-GTP complexes form [2] and aggregate [3] into long, hollow cylinders called microtu bules. Microtubules are about 25 nm in outer diameter, incorporate 1625 tubulin dimers per micrometer of their length [4] ...