Effect of air or 95% O2 on proliferation (BrdU labeling index) in type II alveolar (A; pro-SP-C-positive) or bronchiolar epithelial cells (B) at P3, P7, and P11, WT vs. p47phox−/−. Data are means of 6–8/group, ± SE, *P < 0.05 vs. WT, **P < 0.05 95% O2 vs. air, for the indicated comparisons.