1. Supplementary Figure 1. Suzuki et al. DAPI/BrdU BrdU 24 hrs 48 hrs 72 hrs

2. Supplementary Figure 2. Suzuki et al. AB

3. MergeH2AX p Supplementary Figure 3. Suzuki et al. CPD MergeH2AX p (6-4) photo MergeH2AX p XRCC1

4. Supplementary Figure 4. Suzuki et al. No UV3 min5 min 10 min20 min0.5 hr

5. Supplementary Table 1 BrdU treatment and foci formation Treatment No. of cells (%) BrdU (+)BrdU (+++)Foci+ BrdU-24 hrs44.3 ± 11.915.1 ± 3.717.3 ± 17.5 BrdU-48 hrs11.1 ± 5.651.4 ± 9.767.8 ± 21.4 BrdU-72 hrs4.1 ± 0.985.9 ± 12.786.8 ± 17.2 BrdU-72 hrs4.5 ± 1.185.7 ± 15.141.2 ± 15.3 + 1 mM APM Exponentially growing cells were labeled with 10 µM BrdU with or without 1 mM L- Ascorbic acid phosphate magnesium salt (APM) for various times as indicated. Cells with weak or strong BrdU signals were counted as BrdU (+) and BrdU (+++) cells, respectively. Approximately 3000 nuclei were examined. Data = mean ± SD.

6. Supplementary Table 2 Effect of BrdU treatment on foci formation Treatment % of cells with phosphorylated H2AX foci Without BrdU2.9 ± 1.1 With BrdU5.7 ± 1.9 Cells were incubated with or without 10 µM BrdU for 72 hours. Data = mean ± SEM

7. Supplementary Table 3 Foci formation in RPA-positive cells Treatment No. of cells (%) RPA+Foci+RPA+/Foci+ Density inhibition 3121 51 (1.6)2787 (89.3) 49/2787 (1.8) Mitotic shake-off 357 7 (2.0) 310 (86.8) 7/310 (2.3) Exponentially growing cells were labeled with 10 µM BrdU for 72 hours. Then, they were subcultured at a high density for 5 days. Or, mitotic cells were collected by tapping the flasks.

8. Supplementary Table 4 Apoptosis induction by UVC exposure UVC doseTotal no. cellsNo. of apoptosis-positive cells (%) (J/m 2 )counted 010008 (0.8) 20-12 hr50017 (3.4) 20-24 hr50057 (11.4) Exponentially growing cells were exposed to UVC without BrdU-labeling and were incubated for 12 or 24 hours. Apoptosis induction was determined by phosphorylated histone H2AX signal, as described by Mukherjee et al.