High-resolution cell division tracking of xenografts in vitro. Xenograft cells were retrieved from cryostorage, labeled with CFSE, sorted, and cultured on MS-5 cells. After 3 days, cells were harvested and analyzed (shaded area) in relation to undivided cells equivalent to day 0 (solid line). Peak channels for undivided (U), division 1 (D1), and division 2 (D2) cells were as indicated. A significant proportion of ALL-7 cells (A) had divided twice by day 3, whereas the majority of ALL-11 cells (B) had divided only once. In contrast, only a small proportion of ALL-17 (C) and ALL-19 (D) cells had undergone a single division at this time.