The proximal promoter contains important regulating sequences. Promoter constructs −2100/+600, −358/+33, −226/+33, and −103/+33 were transfected into HL-60 cells. Empty vector pBLCAT3 was used as a negative control. All constructs have high promoter activity, indicating that positively regulating elements are located within the first 103 bp of the proximal promoter. Values are the mean of at least four independent experiments; the error bars indicate the standard deviation. LacZ determination was used to correct for transfection efficiency. No difference was observed between HL-60 and U937 cells (not shown).