Understanding the quantitative principles underlying the durations of each of the four cell cycle phases has remained a challenge, despite the extensive knowledge on the molecular components and mechanisms related to cell cycle control. In their recent study, Purvis and colleagues (Chao et al, 2019) quantify cell cycle phase durations in human cells and propose a model whereby cell cycle progression in single cells is a succession of uncoupled, memoryless phases, each composed of a characteristic rate and number of steps.