Relative activity of γ-globin HPFH promoter plus enhancer plasmids in uninduced K562 cells. Activities are calculated relative to the wild-type γ-globin promoter-luciferase plasmid without enhancer. The activity values are presented as the mean ± SEM and n denotes the number of assays performed. A value of 1.0x represents 45,947 ± 7,860 light units per minute per mg protein per unit SEAP activity.