We show how mitotic waves initiate at pacemakers, regions which oscillate faster than their surroundings. In cell-free extracts of Xenopus laevis eggs, we find that nuclei define such pacemakers by concentrating cell cycle regulators. While Felix developed computational models to illustrate how multiple nuclei can collectively determine the pacemaker location, the experiments were carried out by Alexandra, Arno and Liliana. Our work provides insight into how nuclei and spatial system dimensions can control local concentrations of regulators, influencing the emergent behavior of mitotic waves. For more information, see the publication in eLife. Congratulations to everyone for this great team effort!