The centrosomes provide organizing centers for the formation of the mitotic spindle and are thus critical for proper chromosome segregation during cell division. Matsumoto and Maller (see the Perspective by Arlot-Bonnemains and Prigent) studied the control of centrosome duplication in an in vitro system derived from Xenopus eggs. They find that increased concentrations of intracellular calcium and consequent activation of the calcium-calmodulin-dependent protein kinase II (CaMKII) are required, which is consistent with oscillations in the concentration of intracellular calcium that occur with each cell cycle. Y. Matsumoto, J. L. Maller, Calcium, calmodulin, and CaMKII requirement for initiation of centrosome duplication in Xenopus egg extracts. Science 295 , 499-502 (2002). [Abstract] [Full Text] Y. Arlot-Bonnemains, C. Prigent, A trigger for centrosome duplication. Science 295 , 455-456 (2002). [Full Text]