G₂/M arrest proceeds in ATM-deficient cells but is inhibited in ATR-deficient cells. (A) Cells were treated with 10 μM tachpyridine for 24 hours and analyzed by flow cytometry. (B) Western blotting confirms absence of ATM in ATM-deficient cells. (C) ATR^flox/- cells were treated with Adeno-Cre at a multiplicity of infection of 10 for 2 days to eliminate ATR expression. Cells were then treated with 10 μM tachpyridine for 24 hours and analyzed by flow cytometry. (D) Western blotting was used to assess ATR levels in HCT 116 parental cells, HCT 116 ATR^flox-/- cells, and HCT 116 ATR^flox-/- cells treated with Adeno-Cre.