Contrary to prevailing models, recent research reveals that cyclin-dependent kinase (CDK) activation triggering mitosis occurs primarily within the cell nucleus rather than at cytoplasmic structures such as the spindle pole body or centrosomes. Mutant analyses demonstrated that premature activation at spindle pole bodies does not advance mitotic entry timing. Additionally, disruption of cyclin B localization to these cytoplasmic sites did not impair nuclear CDK activation. These findings reshape our understanding of the spatial control of mitotic onset and highlight the critical role of nuclear CDK dynamics.