Human keratinocytes were transfected by nucleofection with siRNA against ATR and/or p53, or a non-targeting/scrambled control (scr). Forty-eight hours later, cells were exposed to 75 mJ/cm2 of UVB. (a) Western blots using the indicated antibodies. Cells were harvested 8 h after UVB irradiation. (b) Percentage of sub-2N DNA content measured by flow cytometry. Cells were harvested 24 h after UVB irradiation and stained with propidium iodide. (c) Relative cell death was calculated by comparing percentage of sub-2N DNA content in ATR/UVB-, p53/UVB-, or ATR+p53/UVB-treated cells with that in scr/UVB-treated cells in each experiment. Average of relative cell death is shown (n=3). Error bar, standard error of the mean.