Fig. 1. Random cycling REF-52 cells do not recover from hydroxyurea or aphidicolin block after 30 hours of exposure. Random cycling REF-52 cells were treated with aphidicolin (10 µM) or hydroxyurea (2 mM) for the indicated times, then released for 24 hours into nocodazole (0.25 µg/ml), and finally subjected to FACscan analysis. Failure of cells exposed to drug for 30 hours or more to replicate upon release is reflected by the absence of cells arrested in 4N by nocodazole.