Critical KLF-binding sites on the HBA promoter. Luciferase activity of wild-type or mutant HBA promoter constructs co-expressed with either KLF2 (A) or KLF4 (B) were observed in bovine aortic endothelial cells (n=3–4 independent experiments). Gray boxes represent mutant KLF-binding sites on the HBA promoter. Data are presented as mean ± standard error of the mean (SEM) values. *p < 0.05 and **p < 0.01 when compared with a co-expression of wild-type HBA promoter with KLF2 or KLF4. KLF2, Kruppel-like factor 2 plasmid; KLF4, Kruppel-like factor 4 plasmid; HBA, hemoglobin subunit alpha; Luc, luciferase reporter; NS, not significant.