When I was an undergraduate student, knowing three key concepts would ensure success in 'Cell Cycle 101': that oscillations of cyclin–cyclin dependent kinase (CDK) complexes drive the cell cycle; that directionality is ensured by the irreversibility of periodic proteasome-mediated cyclin degradation; and that checkpoints ensure the dependence of later cell cycle steps on the completion of earlier events. A recent study by Coudreuse and Nurse now suggests that only the first concept may be fundamentally essential for cell cycle progression.