(a) A549 cells were exposed mock transfected or with siRNA oligonucleotides against luciferase, ATM, hSMG-1, or ATR and immediately harvested (0 hours) or after 48 hours of hyperoxia. Cell lysates were immunoblotted for p21 and actin. (b) Representative DNA histograms of cells transfected with siRNA oligonucleotides against luciferase, ATM, hSMG-1, or ATR and exposed to hyperoxia for 48 hours. (c) The percentage of transfected cells in S phase was determined and graphed. Values represent mean % +/− SEM (n = 4 experiments). (d) A549 cells were cultured in room air or hyperoxia for 48 hours and then treated with cyclohexamide. Cells were immediately harvested in duplicate and after one hour. Lysates were immunoblotted for p21 and actin.