Ran, a small guanosine triphosphatase, is suggested to have additional functions beyond its well-characterized role in nuclear trafficking, Guanosine triphosphate-bound Ran, but not guanosine biphosphate-bound Ran, stimulated polymerization of astral microtubules from centrosomes assembled on Xenopus sperm, Moreover, a Ran allele with a mutation in the effector domain (RanL43E) induced the formation of microtubule asters and spindle assembly, in the absence of sperm nuclei, in a gamma TuRC (gamma-tubulin ring complex)- and XMAP215 (Xenopus microtubule associated protein)-dependent manner. Therefore, Ran could be a key signaling molecule regulating microtubule polymerization during mitosis.