By comparing the expression of Nrf2 and HO-1 in the lung tissues of the control group and the BPD group using western blotting, we found that at P5, the Nrf2 expression in the BPD group was significantly downregulated, but the expression of HO-1 was increased (all P < 0.05) (Fig. 2a, b), suggesting an imbalance in the antioxidant stress system. This evidence concerns the gene HMOX1 and bronchopulmonary dysplasia.