Resistance to RS in Chk1^Tg MEF. (A) Number of colonies per plate in WT and Chk1^Tg MEF after treatment with 0.5 mM HU or 1 µM APH for 24 h (normalized to the number of colonies found in the untreated control plates). The experiment was repeated twice with three independent MEF pairs. (B) Representative flow cytometry profiles of WT and Chk1^Tg MEF after a 1-h BrdU pulse. Numbers indicate the percentage of BrdU-positive cells. Data are representative of five independent analyses. (C and D) HTM-mediated quantification of γH2AX (C) and RPA (D) intensities in WT and Chk1^Tg MEF treated with 0.5 mM HU for 3 h and 5 µM APH for 4 h. Data are representative of four independent analyses. In A, error bars indicate SD; in C and D, center lines indicate mean values. *, P < 0.05; **, P < 0.01; ***, P < 0.001.