KLF9 and bronchopulmonary dysplasia: We subsequently assessed gene expression changes in BPD based on multiple physiologic and clinical definitions and were successful in identifying nine genes (GPCPD1, MTSS1L, USP15, DDX24, KLF9, CLK1, ZC3H7A, ITCH, and PIK3R1) that were consistently different, and upregulated in BPD, irrespective of definitions, or analytical approaches (Supplemental Figure 4).