Fig. 2.

H2B-FT reports live cell cycle speed heterogeneity in snapshot measurements. (A) H2B tagged with a color-changing fluorescent timer (FT) protein reports cell cycle speed heterogeneity due to the divergent half-lives of the nascent (blue) form as compared to the mature (red) form. While mitosis halves cellular levels of both the blue and red species, the red form’s greater stability allows it to accumulate while the short-lived blue form cannot, making red levels sensitive to modulation by cell division frequency. Faster-cycling cells are therefore distinguishable from slower- or nondividing cells by their higher intranuclear blue/red fluorescence ratio. (B) Cells with distinct proliferation rates can be identified in a two-color fluorescence assay, such as flow cytometry.