Cell-cycle progression and lineage commitment of stem cells seem to be tightly linked but it has not been possible to study this relationship in live stem cells. Now, Matthias Lutolf and colleagues () describe a single-cell tracking approach that enables the automatic detection of cell-cycle phases in live stem cells that express fluorescent ubiquitylation-based cell-cycle indicator (FUCCI) probes. The researchers use their approach to identify distinctive changes in the length of cell-cycle phases and in the fluorescence intensity of the G1 (red) and S/G2-M (green) FUCCI probes during the differentiation of adult neural stem/progenitor cells (NSCs) and embryonic stem cells. Moreover, they use these changes in fluorescence intensity to purify NSCs from a heterogeneous population and to increase the proportion of reprogrammed cells obtained during NSC reprogramming to an induced pluripotent stem cell-like state. These findings shed new light on the relationship between cell-cycle progression and cell fate choice and introduce a tool that could advance our understanding of stem cell biology.
