Fig. 5. Human non-transformed IMR-90 fibroblasts permanently arrest following prolonged S phase arrest, as indicated by FACScan analysis of random cycling cells 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). In parallel with results from REF-52, IMR-90 cells released from G0 arrest require longer than random cycling cells to permanently arrest (data not shown).