Chk1 and Chk2 phosphorylation correlates with G₂arrest. The indicated cell lines were cultured for 24 hours in the presence or absence of the indicated concentrations of GO. Cytoplasmic extracts (60 μg) were analyzed by Western blotting using antibodies specific for Chk1 protein, Chk1 phosphorylated on Ser345, Chk2, and p21. The results are representative of at least 2 independent experiments.