Fig. 3. REF-52 cells released from G0 state require longer drug exposure than random cycling cells to permanently arrest. REF-52 cells were synchronized in G0, as described in Materials and Methods, treated with aphidicolin (10 µM) or hydroxyurea (2 mM) for the indicated times, and then released for 24 hours into nocodazole (0.25 µg/ml). FACScan analysis shows failure to replicate after at least 45 hours of S phase arrest.