(a) H1299-EGFp21 cells were transfected with siRNA oligonucleotides against luciferase, ATM, ATR, or hSMG-1 and then exposed to hyperoxia and doxycycline for 24 hours. Cycloheximide (50 µ g/ml) was added to inhibit new protein synthesis and cells immediately harvested (0 hours) or 9 hours later. Lysates were immunblotted for EGFp21 and actin. (b) The percent of EGFp21 remaining 9 hours after cycloheximide was determined for each treatment and graphed. Values represent mean +/− SEM (n = 3 experiments). (c). Representative DNA histograms of H1299-EGFp21 cells transfected with siRNA oligonucleotides against luciferase, ATM, hSMG-1, or ATR and exposed to hyperoxia for 48 hours in the presence (+dox) or absence (−dox) of doxycycline. (d) The percentage of cells in S phase was determined and graphed. Values represent mean % +/− SEM (n = 4 experiments).