Rac1 inhibition abolishes IR-induced activation of MEK1/2 and ERK1/2. (A) MCF-7 cells were incubated with increasing doses of NSC23766 for 1 hour at 37°C, exposed to 20-Gy IR, and incubated for additional 15 minutes. The cells were analyzed with immunoblotting for levels of phospho-MEK1/2 (pMEK1/2), phospho-ERK1/2 (pERK1/2), total MEK1/2 (MEK1/2), total ERK1/2 (ERK1/2), and Actin (Actin). (B) Cells were transfected with control or Rac1 siRNA, incubated for 2 days, and then exposed to 20-Gy IR or left nonirradiated. After incubation for 15 minutes at 37°C, the cells were examined for levels of phospho-MEK1/2, phospho-ERK1/2, total MEK1/2, total ERK1/2, and actin.