Inhibition of Rac1 sensitizes MCF-7 cells to irradiation. (A) MCF-7 cells were exposed to increasing doses of IR in the presence or absence of 100 μM NSC23766 and incubated for 3 hours after IR. The cells were washed, incubated in regular growth medium for 7 days, and analyzed for viable cells with crystal violet staining. Left panel: representative sample dishes stained with crystal violet. Right panel: amount of viable cells in each sample was quantified with the ImageJ analytic program, and the results expressed as the mean ± SD of triplicate samples (right panel). * P = < 0.001 (n = 3). (B) A parallel set of cell samples from this treatment were photographed by using phase-contrast optics. (C) Upper panel: cells were exposed to IR at the indicated doses, incubated for 3 days, and analyzed for levels of full-length PARP by immunoblotting (PARP). As a control, Actin levels in cell lysates were assessed with immunoblotting (actin). Lower panel: cells exposed to 10-Gy IR in the presence or absence of NSC23766 were incubated for 3 days and analyzed for levels of full-length PARP (PARP) and actin (actin) with immunoblotting.