Luciferase activities in −0.15 TG and −4.0 TG mice suggest elements 5′ of the proximal promoter are necessary for enhanced erythroid expression. Mice containing the −0.15 TG and −4.0 TG constructs were treated with a 7-day regimen of PBS (PHZ-, hatched bars) or PHZ (solid bars). On day 9, fragments of liver or spleen were homogenized in luciferase assay buffer and luciferase activity was assessed as described previously. Letters below each set of bars represent individual founder mice. Each bar represents an average luciferase activity of two to three mice from each founder. Three scales are necessary to accurately display the range of data obtained for the two transgenic lines.