The DiI-tracing assay showed that tumor cells exhibited clear heterogeneity with respect to cell-cycle length. (A) DiI-labeled CT-26 cells were cultured in complete RPMI 1640 medium on day 1, and the percentage of DiI-retaining cells was analyzed on days 1, 3, 5, and 8 by flow cytometry. (B) DiI-labeled CT-26 cells were injected subcutaneously into mice on day 1. On days 10, 15 and 25, tumors were excised and digested, and the percentage of DiI-retaining cells was analyzed by flow cytometry. Experiments were repeated twice with similar results.