B, ATR is required for BIR/RMD upon cyclin E overexpression. RMD frequency was determined in U2OS (EGFP-SSA-3427-3) cells infected with lentivirus-expressed ATR shRNA or the control vector (Ctrl) with and without induction of cyclin E expression by DOX (0.5 µg/ml) after R29 sgRNA/Cas9 cleavage. ATR knockdown was shown by Western blot with KU70 as the loading control. 