Ki67 is a graded rather than a binary marker of proliferation vs. quiescence

Ki67 staining is widely used as a proliferation indicator in the clinic, despite poor understanding of this protein’s function or dynamics. Here we track Ki67 levels under endogenous control in single cells over time and find that Ki67 accumulation only occurs during S, G2, and M phases. Ki67 is degraded continuously in G1 and G0 phases, regardless of the cause of entry into G0/quiescence. Consequently, the level of Ki67 during G0 and G1 in individual cells is highly heterogeneous and depends on how long an individual cell spent in G0 prior to cell cycle re-entry. Thus, Ki67 is a graded rather than a binary marker both for cell-cycle progression and depth of quiescence.